On the outside, they may be Human Aliens, completely indistinguishable from us Puny Earthlings. Or they can be shapeshifters. Or Rubber-Forehead Aliens. Whatever...
What is the best way to quickly establish that a character is not human, sometimes to shock other characters around? Dissection? Squick. They may have Bizarre Alien Biology, but that's usually just an informed characteristic which is rarely appealed to, if ever. Unless it's something very obvious, like having two hearts. Solution? Make them bleed. If their blood (assuming it is actually blood and doesn't serve an entirely different purpose in the body) is not red, or nonexistent at all, they might as well have the words "NOT HUMAN" written on their forehead.
The trope, of course, is not limited to Human Aliens (Plant Aliens often have green blood, for no apparent reason), but for other types, it doesn't allow for The Reveal in such an effective way.
With Human Aliens, this is a very mild case of Artistic License - Biology: Since the skin is translucent, a creature with different-colored blood wouldn't have the same skin color as a human...if the skin is white. Darker-skinned aliens, or ones with scales, feathers, hair, or something else hiding their skins avert this problem, albeit probably unintentionally.
Of course, here on Earth, all vertebrates (with the exception of certain lizards and genetic conditions), the largest and most complex members of the animal kingdom, have red blood—the reason? Because all animals breathe oxygen. Red blood cells are red because they contain a protein chemical called hemoglobin, which is bright red in color. This is because hemoglobin contains iron, which is vital for transporting life-sustaining oxygen where it needs to go. There are other pigments like hemocyanin or chlorocurorin that come in such vibrant shades as pink, orange, blue, and green, but these are much less efficient at transporting oxygen than hemoglobin and thus only appear in invertebrates, which have smaller bodies and a better surface-area-to-volume ratio, and thus less need of efficient oxygen transport. Usually hand-waved by having different chemicals in their blood.
In Video Games, Aliens and Monsters often have different colors of blood than red, frequently as an optional mode, to appease the Moral Guardians. Because it's okay to shoot non-human beings if they don't bleed the same way.
Assuming it's not red like human blood, Aliens and Monsters most commonly have blood that's either black or some bright color usually associated with something unhealthy, like orange, green, or yellow (blue and violet are possible, but rare). Robots and Cyborgs, on the other hand, are often shown squirting white fluid when damaged (or black, which is arguably oil). The white blood can be traced back to the android in the movie Alien, who was practically overflowing with the stuff and very squishy compared to most robots in film at the time. In a case of reality imitating fiction perflourocarbon based blood substitutes are white and carry oxygen much more efficiently compared to red blood cells.
Not to be confused with Blue Blood, even if it is, in fact, blue. (Though some blue-blooded aliens can, in fact, be Blue Bloods.) Also not to be confused with a book of same name by Joan D. Vinge.
Partially subverted in Tenchi Muyo! when Tenchi was surprised to find that alien princess Aeka had red blood. Villainous Kagato, on the other hand, bled green.
In Claymore, the shape shifting, human-eating yoma have purple blood. The titular half-yoma warriors have red blood, normally, unless they Awaken, permanently turning into their Superpowered Evil Side. From then on, they'll bleed purple.
While not aliens or monsters, transformed wielders from Witchblade would bleed clear crystalline blood.
Judging from the scenes where Nephrite is bleeding and dying, the Dark Kingdom Generals from the first season of Sailor Moon have green blood, even though they have normal human skin tones and the insides of their mouths are pink (as can clearly be seen on any of the numerous occasions when they Evil Laugh).
Zoisite, at least, has been shown to have normal red blood (in episode 34 Mamoru injures him with his rose). Nephrite is apparently somewhat of a case of censorship.
Kunzite appears to have hot pink blood (in the episode where a Crescent Beam cuts his hands). Perhaps he's a Klingon?
In a rather bizarre reversal of this, the aliens Eiru and An from the first arc of the second season have red blood, even though they have green skin and the insides of their mouths are clearly also green.
The androids in the Ghost In The Shell series consistently have white blood, a possible reference to the Alien franchise (See below)
Magic-users in Dorohedoro have black particles mixed in with their blood.
The Aragami of Blue Seed, being essentially plants, have green blood; this extends to the half-Aragami mutants like Kusanagi, though the latter looks extraordinarily enough not to be mistaken for a normal human. (The color of his blood still helps, considering that he tends to get injured a lot.)
In the Hentai anime La Blue Girl, Shikima demons have blue blood; this is also the case of the heroine, Miko Mido (hence the title), since she is half-Shikima.
Dragonball Z has a few other instances as well. Most villains bled purple or dark red, but occasionally you'd see something weirder. For instance, Zarbon (one of Frieza's henchmen) bled dark blue, which was also reflected in his skin color.
On a side note, there was at least one instance of a retconned blood color: Piccolo's blood was red in an early DBZ episode, censored to light green in the dub. It later became purple and was no longer censored.
In Dragonball Kin Piccolo bled red, but this was when he was portrayed as a demon rather than an alien.
Most of the Boomers in Bubblegum Crisis tended to bleed bright orange fluid when damaged, which is unusual for robotic enemies.
Non-alien example from Naruto: Tobi bleeds white...stuff. Going by future revelations, it seems to be a sort of human flesh/plant matter hybrid, similar to what makes up the Zetsu clones.
The impetus for the war between two groups of people in "Seiketsu no Hagurama", with one side having red blood and the other with blue blood. The main character and Gadgeteer Genius is a blue-blooded prince who discovers to his horror that all his steam-punk looking inventions have been used to eradicate the remaining refugee red bloods by orders of his father and king.
In Mahou Sensei Negima!, magical constructs built by the Lifemaker such as the Averruncus series have blood that is white or transparent. Given the way they come into being and what happens when they die, it's likely that they are essentially built out of pure magic and are actually bleeding magical energy.
In the (now closed) comics Gea the theme is that extradimentional beings lives on the Earth and are the basis of every myth or religion. As in Men in Black, they try to live under cover, enforced by the Guardians ("Baluardi" in the original), such as the main character. Even when outsiders are outwardly identical to humans (not so common), expect them to be killed in a gore splash of weird blood and alien organs. In one particular case, a team of racist vigilantes kills a group of "outsiders" living in a slum, and are horrified to see that they bleed yellow, smelly liquids. The leader opts for a "foreign disease" and orders the bodies to be burnt.
In Diaries Of A Madman changeling blood will always be green, no matter what species they're currently posing as. The protagonist of the story, Navarone, also gains this after his Emergency Transformation, going from regular human blood to a more viscous amber blood.
One of the princes in Stardust has literal Blue Blood. Unknown if it's just because he's royalty, or if all inhabitants of the magical world have such blood, since nobody else in the movie is seen bleeding. Word Of God states the decision for the color was made to avoid an "R" rating, with the added benefit of being a visual gag for the character.
The Xenomorphs in Alien have yellow, highly acidic blood that can burn through steel, while the androids have milky white fluid inside them instead of blood. In the later movies, the blood is green instead, possibly as a result of morphing with human DNA. A reasonable theory, however would be that the acid doesn't actually have the same purpose as blood- it's more of a defence mechanism that makes its victims reluctant to kill it.
The Predators from the films of the same name have fluorescent yellow/green blood. As Schwarzenegger famously says in the first film:
If it bleeds, we can kill it.
Not quite aliens, but one of the first hints that the employees of the Titty Twister in From Dusk Till Dawn aren't human is a close-up of a knife which was just used to stab one of them. Instead of blood, it's covered in a translucent goo.
In John Carpenter's The Thing (1982), blood samples are taken from all the characters and exposed to heat to test which one is the shapeshifting alien. The alien blood, it turns out, is capable of leaping around the room as an independent organism. The character who proposed and carried out the test did it because he figured that the blood might well do that, as severed pieces had previously been observed operating independently.
The monster Gamera has green blood. Not that it's ever likely to be useful as The Reveal, but notable anyhow for the sheer volume you will be seeing in any given movie — that turtle bleeds like a Garbage Wrestler...
Karl Ruprect Kroenen from Hellboy has sand instead of blood. He's not an alien, but a Nazi cyborgzombie who's been through more than Rasputin. It's simply the years of abuse and being in a state of undeath that caused his blood to dry up.
Also in the Hellboy movies (and animated movies, but not the comics), Abe Sapien has blue blood.
In Hellboy II, the elven royalty has greyish yellow blood.
The Moorwen from Outlander has bright green blood.
The Strangers from Dark City melt into goo or turn red and dissolve into flakes when they die; the dead human bodies they use as vessels have black blood.
The Tall Man in Phantasm gets his hand caught in a door at one point, and the boy cuts off his fingers with a blade. They bleed yellow; the director noted on the commentary that most test audiences were shocked at the unusual color.
In Demon Knight The Collector and his minions' blood is green.
Buckets of black and green Deadite blood get thrown around in the Evil Dead movies.
In the Disney John Carter Martians have blue blood. Despite all references to their blood in the books indicating that it was red.
The flying monsters from Pitch Black have bluish-gray blood.
Averted in District 9, where the Insectoid Aliens bleed a scarlet red. Possibly done to show that despite their appearance, they are not monstrous at all. Zigzagged with Wikus, who at one point has a nosebleed of a black fluid and vomits up black goo, but still bleeds red.
The Kaiju from Pacific Rim bleed blue when stabbed, slashed, gouged, plasma blasted, or otherwise maimed. Which happens a lot. Too bad the blood's toxic as all hell, though.
In The World's End, the alien robot replacements for the townspeople all have blue "blood".
The aliens from The Green Slime: killing them is a bad idea because the puddles of light green blood will eventually regenerate into numerous more creatures.
Whenever aliens are blown up in the Men in Black films we see strangely-colored fluids. Mikey's blood is bright blue. Jack Jeebs' is green.
The mutants from the future in the Mindwarp kid lit series had metallic silver blood.
Many dragons, in all forms of media, have boiling blood, including in the The Malloreon.
The demons in Armageddon:The Salvation War have yellow, green, and acid blood. Not an instance of the Rule of Cool, however. They've been evolving in a very different environment for the past 5 million years. Quite possibly the colors have something to do with the environment...
The angels in the sequel have silver or white blood. For both species, blood color was observed to function like blood groups in humans for purposes of transfusions.
Humanity in Robert Reed's Great Ship universe have very dark red, almost black blood due to the huge amount of genetic engineering and extra bits of DNA and cells in their bloodstream. The Remoras, twisted by radiation and their own tailored viruses, often have odd blood - one of the characters, Orleans, has very thick black blood.
CDF soldiers in Old Man's War have their blood replaced with SmartBlood, a gray nanotech fluid that carries 4x the oxygen that human blood can and which can instantly seal wounds and even be commanded to ignite remotely in case something drinks your blood (or if you spit some of it in an attacker's eyes).
The Martians from The War of the Worlds are a strange case in that they have no blood. And no digestive system. They obtain oxygen and nutrients by removing blood from other species and injecting into their own bodies.
Vulcans in Star Trek have green copper-based blood, not unlike Earth insectsnote In reality, Cyanoglobin (the oxygen-carrying pigment that is the copper-blood equivalent of iron-based haemoglobin) does not carry oxygen very efficiently, which is why molluscs like squids and octopi tend to operate on a 'dart and rest' principle.. In the movies, Klingons have pink blood. Ironically, Star Trek VI, which introduced the Klingon Pepto-Bismol Blood, shows a Klingon bleeding red blood in a climactic scene - at least in the theatrical version. Video releases restored a scene cut from the theatrical release explaining this (the "Klingon" is really a human wearing a rubber forehead).
To a limited extent, Star Trek did Shown Their Work in that Leonard Nimoy's make-up did tend towards a more greenish cast than everyone else's. It did take several tries, however, to get this to work the way they wanted when combined with the lighting and the other make-up, which occasionally had hilarious results—especially in early episodes of the show.
The shapeshifters of Star Trek: Deep Space Nine once caused some paranoia over their infiltration capabilities. As such, Starfleet briefly initiated required blood tests of officers and their family, as any of the shapeshifters' substance removed from their body would instantly revert to their protoplasm. Reassuring? Not quite, as Captain Sisko's father proceeds to point out that if he were a shapeshifter, he'd just kill some poor shlub and use their blood to get around the test. The fact the idea is introduced by someone who later proved to be a changeling infiltrator didn't help.
The Andorians have blue blood, which is just icing on the cake since they also have blue skin and antennae. Bolians also have blue blood (and, again, blue skin).
While the pigment in Vulcan and Romulan blood may use copper similarly to how human blood uses iron, the pigment would not be all that much like hemoglobin. The usual color of copper-based respiratory pigments in Earth life (which are similar to hemoglobin) is blue.
Cardassian blood is black. This also applies to other animal life native to Cardassia Prime, such as the raw fertilized taspar egg Picard ate in "Chain of Command".
Parodied in a comic by Mark Parisi (Off the Mark, August 4th 1996), which shows a movie theater on planet Vulcan. The father reassures his son that what he sees on the screen is not real blood, just guacamole.
In the Buffy the Vampire Slayer episode "Earshot", a mouthless demon's fluorescent blood splashes onto Buffy's hand and gives her an "aspect of the demon", which turns out to be hearing people's thoughts.
Similarly, in Angel, the title character is "infected" with demonic blood that cures him of his vampirism. It's bright neon green.
Power Rangers: A reversal occurs when a villain's human nature is revealed when he does bleed human blood. There was also something of a straight example with Lipsyncher, who "bled" lipstick when her cheek was scarred by Jason's sword.
This happened a couple of times in Brimstone, where someone Stone suspected of being one of the 113, was revealed to be human because they bled.
The alien human hybrids on The X-Files bled green blood that could produce a deadly gas (to non-aliens) when exposed to the atmosphere, as did the aliens themselves. This got lampshaded in a heartbreakingly awesome way in "The Unnatural". Ironically, this means that Mulder could see an alien-human hybrid bleeding, and not know there was anything weird going on because he's red/green colorblind!
An episode of Stargate SG-1 has the team realize something is amiss when an attempt to draw blood from O'Neill yields silvery-white fluid instead.
While the alien Clark Kent bleeds red blood on the few occasions he's hurt enough to bleed in Smallville, cyborg human Victor Stone (Cyborg of Teen Titans fame) bleeds black mech fluids, keeping in line with the comics. This does not carry over to the Teen Titans cartoon, where it appears to be clear.
The alien teenagers of Roswell have both blood and saliva that looks normal to the naked eye but not under a microscope.
In the TV series spin-off of V the distinctive reverberation of the Visitor's voices was dropped due to the expense of sound dubbing, and the Visitors were given cliché green blood instead. Some viewers missed the plot point about a human who'd cut his hand actually being a Visitor infiltrator, as they lacked a colour television at the time.
Luxans in Farscape have clear blood which turns to black when they get hurt. Black blood is toxic, thus the bleeding has to be stimulated until it turns clear again. Also in Farscape, Nebari have blue blood.
Kamen Rider Blade has the Undead, who bleed green. This serves as a Chekhov's Gun as one of the main characters is an Undead himself and another main character becomes an Undead in the finale, and they both bled this color.
Doyle, an android in Andromeda "bleeds" superconductor fluid when cut. Due to her in-built Weirdness Censor, she sees it as ordinary blood, thinking herself human.
Time Lords from Doctor Who (at least in the Expanded Universe) have blood that's a deep orange. It's sometimes described as basically red, but "orange-ish".
In "State of Decay", the Doctor's blood is blue. In every other episode of the series where he bleeds, it's normal red.
In the movie it's red, but is different enough from human blood on a cellular level that Grace describes it as "not blood" when she looks at it under a microscope.
Several other examples from Doctor Who: the Sontarans have green blood, as seen in "The Two Doctors". As does Mr. Sweet from the new series' "The Crimson Horror". The novels follow up with the trend, for instance: the Venusians in Venusian Lullaby have blue blood.
Strangely, a scene from "Warriors of the Deep", episode 4, indicates that the Sea Devils have green blood. This is odd because this particular species is not alien, but rather a race of intelligent Earth reptiles. Perhaps their case is similar to the real life green-blooded skink (Prasinohaema virens)?
The Daleks, as well as Davros himself, all have green blood. It can then be assumed that the rest of the Kaleds, before their mutation, also had green blood as well.
Babylon 5: The Drazi have white blood. On the other hand, the other major races — the Minbari, the Centauri, and the Narns — have red blood.
Perhaps as an intentional aversion of the 'Human Aliens have coloured blood and pink skin' error, the Tau do not look like humans partly because they have deep blue-purple blood (their equivalent of haemoglobin uses cobalt instead of iron), which has the expected effect on their skin tone. In one of the Ciaphas Cain novels, Amberley Veil notes that it smells horrible.
The Orks, who are in fact genetically-engineered carnivorous space-fungi, have green skin but red blood (and mouths). It wasn't always this way and earlier sourcebooks for the game described Ork blood as green, but eventually red gore was decided on. This was explained in White Dwarf magazine with an in-universe biologist's report: while Orks' dense green skin is a chlorophyll- and spore-producing layer, the rest of their body is relatively normal meat, and they have a normal (if supercharged) food-based metabolism to support it. On the same page, the game developers gave the real reason: green injuries on green-skinned aliens were hard to notice, and made the models look like "they've had an accident eating a gooseberry squishy".
Some sources have indicated that the Orks' blood does double duty as their equivalent of a lymphatic system, and that the colour of the blood will as a result vary depending on their diet, ranging from bright crimson to almost black.
The Eldar have red blood like humans, but it crystallizes instead of forming scabs.
The most detailed list of recent editions can be found in the Tome of Corruption for Warhammer Fantasy Roleplay, which details twenty different kinds of blood replacement.
Acid, Electricity, Fire, Molten Metal and Poison, obviously, are all lethal to your assailant.
There are several variants where "blood" equates to "living animals" — centipedes, ants, beetles, leeches, maggots, worms, even mice and small birds. In all cases, these attack the person who wounded you, usually causing minor damage and fighting penalties (birds just cause penalties and mice cause just damage).
Excrement (you do not want details) makes you so disgusting that your enemies find it harder to fight you.
Eyeballs give you a tactical advantage because you can still see through them.
Glue can temporarily stick an enemy against you.
Mucous, Mud or Wax are disgusting, but harmless.
Protoplasm means that if you die, a Blob Monster will emerge and attack your killers.
Tar will spurt out and make your enemy all sticky, impairing their mobility.
Vines will rip into enemy flesh, harming them and impairing movement.
Vomit is not only disgusting, but induces mutations.
Wind is deafening and eerie.
Bungie seems to do this quite a bit:
In the Halo universe, elites and jackals have purple blood, hunters orange, brutes and prophets red, grunts luminous blue, and drones whitish green. The Flood are a different story: they don't bleed just blood; Infection Forms will liquefy the internal organs of the host, so in reality they're spilling out their entrails in liquid form. Unless the "Grunt Birthday Party" skull is enabled, in which case headshots will cause the grunts to bleed confetti and cheering.
In the Marathon series, Pfhor, ticks, lookers and simulacra have yellow blood, hulks blue, S'pht green, and wasps purple.
SaGa Frontier- Mystics have blue blood, Half-mystic protagonist Asellus has purple.
In the Mortal Kombat games, Reptile, Khameleon and (until recently) Sheeva have green blood. Cyborgs have black oil instead of blood, Blaze has molten lava, Noob Saibot has black... wraith juice and Cyber Sub-Zero bleeds a light blue combination of oil and antifreeze.
A major plot point in Star Ocean 1 is that Fellpool have copper-based blood - but everyone from that planet has properly pink skin (or tan, in the case of the Highlanders). The only outward difference from Terrans is their tails and sometimes fangs.
In Mass Effect, a common taunt directed at humans who express conciliatory philosophy toward the varied races of the galaxy is "Do you remember what color your blood is?" Just for fun, stand up to the leader of the isolationist "Earth First (we can exploit the other planets later)" Terra-Firma party.
In spite of this, the player doesn't actually see a lot of oddly-colored alien blood in the game, but the Krogan do bleed bright orange and the robotic Geth "bleed" white fluid. The Dr. Saleon sidequest mentions a few other colors in a bit of flavor text at the conclusion of the mission - "pale blue, orange, violet, and more than a few dark red."
Thorian creepers don't bleed per se, but they do generate splatters of brownish-green material when they die. There's also a scene where Shepard has the option to Mercy Kill a room of Salarians, revealing green blood.
Blood colors in ME also qualify for Shown Their Work credit, as species with alternate colors of blood have skin and/or mucous membrane coloration to match (asari, who bleed purple, have various tones of blue or purple skin as well as lightly blue-tinged sclerae; krogan, who bleed orange, can exhibit red or yellow eyes; Garrus and several other light-colored turians have a faint purplish cast to their facial features).
The krogan have another odd variant on blood, as well. They don't actually have a nervous system made up of nerve cells; rather, they have a second circulatory system consisting of an electrically conducive fluid that runs through their body. As a result, krogan cannot be paralyzed. This physical trait was evolved by just about every species on Tuchanka, the krogan's homeworld.
Shown in practice in Mass Effect 2, when Garrus gets shot during his recruit mission, and bleeds blue all over the floor, and is later seen with purple scar-tissue over third of his face, mostly covered with cybernetics.
In Metal Gear Solid 4, Super Soldier Raiden has white nanomachine-based blood, the potential symbolism of which is not lost to Vamp. This is a reference to Policenauts, where the "Frozeners" also have white artificial blood, called "first-generation" blood, which is said to be simpler than the "third-generation" blood used on Home (Earth). It's also said to transport oxygen more efficiently than regular blood, making blood loss less of a threat.
In Metal Gear Rising: Revengeance, cyborgs use a more advanced artificial blood that is red (except in the Japanese version, where it is still white due to censorship) that does not require the frequent dialysis that the original white blood did.
In the original version of The Legend of Zelda: Ocarina of Time (predominately copies of the game with a gold cartridge), Ganondorf coughed up regular ol' red blood after you defeated him. However, Moral Guardians protested that this was too graphic for an E-rated game, and Ganondorf's blood was changed to green for subsequent releases. Since then, even this trope began to be averted (no blood at all) in the following games, with the exception of Twilight Princess and Skyward Sword (which were consequentially rated T and E10+ respectively for "Animated Blood").
Orcs from Warcraft (even though they are native to a planet called 'Draenor') have very dark blood, which is why many of the more or less racist humans call them blackbloods. Similarly, the blue skinned draenei have blue blood. That, and their unguligrade appearance has caused a few fans to compare them to the above mentioned Warhammer 40k Tau (although people are even more likely to compare draenei to Protoss). Demon blood seem to be fluorescent green (which is pretty much the trademark colour of fel energy). Sufficiently powerful demons have lava instead of blood.
The ant-like enemies found in Jet Force Gemini bleed green blood when you shoot them; and a special "cheat code" makes them bleed rainbow colored blood.
The Necris of Unreal Tournament have black blood, though it's not so much of blood as it is a solution of nanomachines they call nanoblack.
The Cacodemons and Barons of Hell in Doom appear to leak blue and green fluids when they die, even though their blood spatter when hit while alive uses the same red color as seen for other creatures in the game. Word Of God says this is because both do have red blood, but their innards are blue/green. However it has been suggested that they simply didn't have a way to color-shift the red blood splatter sprites and did not want to waste space on differently-colored graphics. Some modern source ports have remedied this by allowing the blood color to be shifted for different enemies.
The Ghost People of the Sierra Madre in Fallout: New Vegas bleed green, but this may not be blood since their insides apparently have a slushy like consistency, so what leaks out of them is the liquid part of the slushy.
In Blood Omen: Legacy of Kain you play a vampire whose health bar is a blood bar, so NPC blood is Color-Coded for Your Convenience: Red blood restores your health, the blue blood of ghosts restores your mana, black blood lowers your health, and green blood poisons you.
Like all things Dwarf Fortress, there are different colors of blood based on the creature that bled it. Which, like all things Dwarf Fortress, can be modded to taste. Someone even made a Homestuck (below) troll mod covering the entire hemospectrum.
The Maians in Perfect Dark have green blood. The Skedar, meanwhile, have red blood.
Discussed in Cthulhu Saves the World: Umi asks the blood types of her allies, and Cthulhu answers he's got ink for blood. Similarly, Ember the demon dragon says to have acid for blood.
In Dra Koi dragons bleed gold. When in human form, this also includes normal human blood.
Resident Evil 3: Nemesis has the Nemesis monster bleed red when shot at, but when he gets knocked out after taking enough damage, his pool of blood is purple.
The trolls of Homestuck have a complex caste system based on the color (thirteen known exist) of their blood. Each troll has a different blood color, spanning the entire color spectrum. The highest castes have literal Royal (Purple) Blood, while the lowest castes have brownish red blood. Trolls higher on the spectrum live longer lives and tend to possess greater physical strength, while rustblood colors tend to have greater psychic powers. Greenbloods (the middle colors) tend to be more emotionally stable. Additionally, Karkat, Kanaya, and Feferi, have rare and special blood-types.
Karkat's blood is a mutation — bright red like human and carapace blood. This puts him completely outside (and beneath) the entire hemospectrum and has only been possessed by one other troll in history, his ancestor The Sufferer. As a result, Karkat opts for "blood anonymity" by typing in grey instead of his blood color as all the other trolls do.
Kanaya has jade blood, which also lets her enjoy Alternia's blistering sunlight and be awake during the day when the undead normally stir, making her Alternia's Goth equivalent. It also marks her as one of the trolls to tend to the mother grub.
Each troll's lusus (a monstrous surrogate parent) shares the same blood color as their adopted troll. In addition, all the Horrorterrors share the same royal purple color Feferi and her lusus do.
Also, for some reason, troll robots and artificial limbs have blood. Equius uses this to give a rustblood he has a crush on a robot body with blue blood befitting his station. This makes her occasionally prone to violent outbursts, like Equius himself.
His Honorable Tyranny, the figurehead monster "in charge" of the Alternian court system has black blood.
Cherubs also have Alien Blood, with the color depending on which personality is dominant. Calliope has lime-colored blood (same name as an exterminated troll caste, but a different color), while her "brother" Caliborn has the same red as humans do.
Carmilla, a student at the Super Hero School Whateley Academy in the webfiction Whateley Universe, is mostly-demon, even if she looks like a hot 'urban vampire' goth kid. She bleeds a blackish-purple ichor which is definitely not blood.
Same school: Eldritch is a mystical Artificer who bleeds a roughly-blood colored stuff that's too high in metals to be anything a human could have in his/her veins.
When the leprechaun in The League of S.T.E.A.M.'s webisode "Fool's Gold" is dispatched, he turns into a pile of green goo.
An episode of Megas XLR featured a shapeshifting plant alien. Once Coop discovered its true nature, he formed one of the robot's limbs into a chainsaw and got to work. The scene of the chainsaw going into the creature while green goop flies out goes on for over thirty seconds.
Kif Kroker from Futurama bleeds viscous green blood. It's tangy!
Agent Bishop (Teenage Mutant Ninja Turtles (2003)) seems to have this as a result of his alien-altered physiology; after he was impaled by a hook in one episode, one could briefly see a thick, silver-colored liquid seeping to his clothes.
A scene near the end of Invader Zim episode "Lice" seems to show that Zim (and by extension, all Irkens) has green blood, as he's skinned and the tissue underneath is green. Of course, considering that Zim's skin is green, it makes sense.
Word Of God says that Irkens have see-through pink blood.
In the Fish Police episode "The Codfather" when the titular character is murdered (or so we're led to believe) his blood is purple.
The Twins from Superjail! have green luminescent blood, as shown in "Combaticus", while their father has a more saturated lime color. This could be artistic license or the colorists having forgotten the glowing detail. Other aliens shown in the series are also shown to bleed in green.
On at least two brief occasions, the Twins have been depicted with red blood (after being crushed in "Combaticus", and with bleeding from their ears and noses in the ending of "Troubles with Triplets") which could either be a coloring error or another artistic choice.
In the Thunder Cats 2011 episode "Ramlak Rising", the Giant Squid/Man-Eating PlantHybrid Monster the Ramlak is shown to have a strange blue ichor dripping from the severed ends of tentacles that Lion-O slices off when it first surfaces from the Sand Sea and attacks. When Lion-O strikes a mortal blow, its revealed that the monster's blood is, appropriately enough, water.
Parodied in the The Simpsons, where Moe is trying to convince a group of kids that he's not a troll because he bleeds red. Then he nicks his finger, and a frustrated Moe sees green blood spurting from the wound.
In Transformers Prime, the life-force of Cybertronians is Energon and it varies from blue, to green, to a purple-pink, and it glows. But when the robots are injured it acts exactly like blood.
While certainly not aliens, most insects bleed light yellow (as mentioned above), some green; crustaceans bleed blue.
Technically, many arthropods don't even have blood. They instead have a fluid called hemolymph, which is inefficient at carrying oxygen compared to vertebrate blood, as the trachea system is generally capable of supporting the oxygen needs of their tiny frames.
The most famous of these is the humble horseshoe crab, with blue copper-based blood. Scientists routinely extract blood from the crabs for all kinds of medical procedures & tests, then release the animals after they've regenerated their blood.
Mollusks tend to have blue blood.
Ditto spiders, who's hemocyanin gives them greenish blue blood.
There is a species of pen shell called Pinna squamosa. What makes it unique is that it is the only mollusc to use manganese to store oxygen, in a protein called pinnaglobin. As a result, it has brown blood.
Inverted with deep-sea tube worms, which have red blood like ours despite being from a taxonomic line in which iron-based blood pigment isn't expected.
The Prasinohaema genus of skinks, in contrast to every other healthy vertebrate, have green blood (as well as green muscles, bone, and everything else). This is caused by levels of biliverdin (bile pigment) in the blood that are so high they would kill any other vertebrate. The name, of course, means "green blood."
Due to having little to no hemoglobin, Antarctic icefish have translucent white blood.
Underwater, red blood appears green below a certain depth.
Sulphur can turn a person's blood green.
Carbon monoxide poisoning turns red blood an even brighter, unnatural-looking shade of red.
A convention of anatomical illustration is that deoxygenated blood vessels are depicted in blue on drawings and models. This sometimes leads people to the false conclusion that deoxygenated blood is blue; in fact, it's a reference to how the outsides of systemic veins, which carry this type of blood, look blue when seen through the paler shades of human skin.
While their actual blood doesn't look different, the bone marrow of a patient with advanced leukemia looks lighter in color than that of a healthy person, due to being overloaded with immature, non-functional white blood cells.
Methemoglobinemia is a genetic disorder that causes heightened levels of methemoglobin, which has a lower affinity for oxygen than hemoglobin, making the blood blue or brown. The most famous cases of methemoglobinemia are the "Blue Fugates", a Kentucky family known for their blue complexion.
The trope of artificial humans having white blood possibly comes from the early research into oxygen-carrying temporary blood replacements for emergency surgery. Some of these work using very different chemical processes to naturally occurring blood (of any colour) and look a lot like milk.
Other scientists are getting in on the fun too, it seems. They have created a oxygen-binding molecule called coboglobin, which is based on cobalt. Like human blood, it changes colors based on if it's oxygenated or not. If it is, then it's clear; if not, it turns amber yellow.