A HairColorSpoiler is a plot twist that is spoiled by bizarre and improbable hair styles often seen in anime. In its most common form, a character is introduced who is related to an existing character. The writers might start dropping hints right away about their relationship, but it doesn't matter -- you already noticed that only those two characters have that exact shade of blue hair -- because YouGottaHaveBlueHair.

For a rather more blatant genetic hint, see ChocolateBaby.

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[[folder: Since this is a spoiler trope, there will be unmarked spoilers below. ]]

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[[folder:Anime and Manga]]
* ''LightNovel/ZeroNoTsukaima'' stretches this out. In season one, we learn about Tabitha's backstory and meet her mother, who has the same exact shade of light blue hair as her. Her butler, MisterExposition, tells us about Tabitha's evil uncle who usurped the throne. The story is dropped until season three, which introduces the mastermind behind the mastermind behind the mastermind of the events of season one. He wears a crown, and his hair color is the exact same shade of blue as Tabitha's and her mother's.
* In ''Anime/YuGiOh'' Season 5, it is supposed to be a huge reveal that Leon Wilson is actually the younger brother of season villain Zigfried von Schroeder. Except that the von Schroeder brothers are the only ''Yu-Gi-Oh!'' characters ever to have ''purple'' hair.
* In ''FasterThanAKiss'', Fumino, at one point, meets a woman who, at the end of the chapter, is revealed to be Shouma's mother/Kazuma's stepmother, completely shocking Fumino...even though the woman looked almost exactly like Shouma, even down to the hairstyle.
* In ''PandoraHearts'', Jack's identity is kept mysterious for some time...except the series pretty heavily uses similar hair/eye colors for families, and Jack has literally Oz's hair except with a braid attached. Little surprise when it's revealed that Oz is his descendant and partial-reincarnation. But then, there was that plot point dropped a while back that the Oz we knew might not be the 'real' Oz Vessalius... and then the recent point about how Oz's body is actually Jack's...
* ''{{Naruto}}'': The Fourth Hokage being Naruto's father was guessed by GenreSavvy readers from the moment he first appeared, mostly due to their similar hair and facial structures.
** And in hindsight, Karin being a member of the Uzumaki clan was foreshadowed by her sharing the red hair that also belonged to Kushina, Naruto's mother.
** Furthermore, [[IKnewIt many fans guessed]] that Tobi was in some way Obito Uchiha even AFTER he claimed to be Madara Uchiha in part because of his identical short, spikey black hair.
* ''GundamAGE'': Is very guilty about this.
** Approximately 75% of the fans knew that Flit and Emily would end up together, mainly because images of Flit's future son, Asemu, show him with blonde hair like Emily and not some shade of purple or blue as would be expected if Yurin was the mother.
** Likewise, fans knew that Asemu and Romary would end up together for different reasons, namely; Flit's future grandson, Kio, is basically a {{Gender Flip}}ped version of Romary and there is ''no way'' in hell that Flit would allow Kio to live if he was biologically fathered by Zeehart, who is a Vagan. For the matter, Zeehart is TheMole, so somehow Romary has no choice (plot-wise) but to end up with Asemu.
** This trope coupled with the Dark-Hound's entire design basis, spoils Captain Ash's identity.
* ''{{Betterman}}'': The heroine's brother has gone missing. The good guys are repeatedly helped by a mysterious man with the same odd and otherwise unique tri-colored hair shock the heroine has. Is it her brother, right? No. It turns out, the mystery man obtained the brother's apperance is by eating a seed that grew from his body.
* In ''SaintSeiya'', people inhabited by a god have hair colors not found in nature. Andromeda Shun has green hair, though this is never remarked upon by anyone. Come the Hades plot arc....
* ''BlackButler'': Combined with eye colors for [[OurAngelsAreDifferent the angels]]. Angela and Ash are the only character to have white hair and PurpleEyes (though Angela's hair is [[TrueBlueFemininity blueish]]). Turns out [[GenderBender they're]] [[{{Hermaphrodite}} the same]] [[VoluntaryShapeshifting being]].
* In ''Manga/AttackOnTitan'', this is a trait several of the Titan Shifters have in regards to their Titan forms. For instance, [[spoiler:Eren's]] Titan form shares his eye and hair color, as does [[spoiler:Annie's]]. [[spoiler:Reiner's]] titan form shares similar build and hair color but [[spoiler:Bertold's]] form is bald and only shares the eye color.
* In ''MaiHime'' the fact that Reito and Mikoto are brother and sister is pretty easy to guess since they have the exact same hair and eye colors, but it takes close to the entire series for a proper [[TheReveal reveal]].
* In the episode of the ''Manga/FairyTail'' anime where Mystogan is unmasked, it blatantly shows tufts of blue hair, a shade which is shared only by Jellal. It then goes a step further by showing he has the exact same tattoo as Jellal around his right eye. This came as a huge frustration to fans of the manga, especially the latter hint, which wasn't even given in the manga, making TheReveal much more surprising.
** The manga also makes the relationship between Jellal and Siegrain painfully obvious. While the anime dismisses their shared information and completely identical appearance by stating that they're twins at the earliest possible point, the manga gives no explanation, and viewers are left with two identical characters who know what one another is up to, as well as the knowledge that one of the characters is able to create temporary magical copies of himself. It's still treated like a big twist when it turns out that Siegrain is Jellal's magical copy.
* In retrospect, in ''SoulEater'' the late-series reveal that Asura and Death the Kid are brothers is hinted at by them both having black hair with white patterns in it.
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[[folder:Film]]
* ''Film/CrazyStupidLove'': Hannah being Cal and Emily's daughter is spoiled due to the mother and daughter having the same red hair.
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[[folder:Literature]]
* In ''Literature/ASongOfIceAndFire'', which house a person belongs to can often be determined simply by their appearance. This becomes a huge plot point in the first book, when Ned realizes that all King Robert's children are blonde, like the Lannister family on their mother's side, as opposed to having the dark hair associated with House Baratheon. He checks the lienage archives and goes to inspect all of Robert's illegitimate children (there are ''tons''), and not a single one is fair-haired. It's crucial to Ned realizing that the King's children are the product of an affair Queen Cersei had with her twin brother Jamie and that one of Ned's close friends Jon Arryn had also discovered this (though despite initial evidence the Lannisters were actually ''[[NotMeThisTime not]]'' the ones behind Arryn's death).
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[[folder:Live-Action TV]]
* In ''Series/PrettyGuardianSailorMoon'', a new senshi appears at the end of an episode, sporting blue cat ears, a blue tail, and blue hair. It is 'revealed' in the next episode that she really is Luna, the, you know, blue-furred cat.
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[[folder:Video Games]]
* In ''VideoGame/ChronoTrigger'', after traveling to the Kingdom of Zeal in 12,000 BC, you meet Queen Zeal and her children, Schala and Janus. The three of them have the exact same shade of light purple hair which is found on nobody else -- except Magus, who is actually an older Janus.
* In ''VideoGame/TheLegendOfDragoon'', the revelation that party-member Meru and penultimate villain Lloyd are both Winglies is spoiled by their platinum (with a blue tint for Meru) hair, which is exclusive to Winglies.
* In ''VideoGame/PokemonBlackAndWhite'', the relation between N and Ghetsis is played as a very dramatic twist. However, more GenreSavvy fans were speculating some sort of relationship before we even knew much of who they were, starting the moment they noticed such close shades of green and distinctive curls/fray/whatever-you-want-to-consider-it.
** This ''despite'' the fact N was adopted.
* ''VideoGame/TalesOfDestiny'': The only party member with hair similar to Rutee's (dark, cut short) is Leon. Guess who's her brother?
* Notably ''avoided'' in ''VideoGame/LastScenario''. A OneDegreeOfSeparation twist is hidden by the fact that one character's hair [[DiseaseBleach changed color]], and by the time that fact is revealed, the player probably won't think to connect them to a random person in someone else's backstory.
* In ''VideoGame/GoldenSun'' games, only Adepts ever have AnimeHair in wacky colors and styles, usually [[ColorCodedElements reflecting their elemental affinity]]. Knowing this, anybody picking up ''VideoGame/GoldenSunDarkDawn'' will immediately identify Amiti (cyan-blue) as a [[MakingASplash Mercury Adept]], and Blados (dark green) and Chalis (pink) as Adepts of some sort.
** Amiti's blue hair is a twofer, since he lives in an area otherwise exclusive to non-Adepts with brown or black hair. This gives away the fairly early reveal that he's [[HeroicBastard not entirely of local stock]].
*** There's an entire ''subplot'' about people noticing that Amiti [[StrongFamilyResemblance kinda looks like]] that one ''other'' [[PaperThinDisguise plot-important bishounen with cyan-blue hair]], and [[WeWouldHaveToldYouBut worrying about whether to point that out to him]].
* The ''VideoGame/FireEmblem'' series is very fond of this trope. A notable example appears in ''[[VideoGame/FireEmblemTellius Radiant Dawn]]'', where the character Pelleas has blue hair and eyes while his mother Almedha has green hair and red eyes (which is an example of the trope in itself for anyone who's played ''Path of Radiance'') and his father, Ashnard, the deceased king of Daein, has black hair and eyes. To the shock of everyone involved (but not any player with the least bit of GenreSavvy), Pelleas turns out to be completely unrelated to the pair; an orphan Almedha started believing was her son out of grief of having lost her real one. Their real son is still alive, incidentally, and has black hair, red eyes and his father's pale skin.
* ''VisualNovel/LittleBusters!'': Although the two girls couldn't be more different in other respects, the fact that Haruka and Kanata have the exact same shade of pink/purple hair - and the fact that they both have identical [[ImportantHairAccessory hair bobbles]] - is a pretty obvious sign from early on that they're related. Naturally, the game treats whatever relationship they might have as a total mystery until the reveal.
* ''[[LufiaAndTheFortressOfDoom Lufia]]'s'' iconic blue hair, which makes it somewhat predictable that her expy Seena from ''LufiaTheLegendReturns'' will have an exact same twist that she's [[HighHeelFaceTurn Erim the Sinistral of Death]] in a disguise.
** Subverted with ''[[VideoGame/LufiaIIRiseOfTheSinistrals Rise of the Sinistrals]]'', however. Tia also has blue hair[[hottip:*:(In the original game at least. The remake gives her orange hair instead.)]], but isn't connected to Lufia or Erim at all.
* In ''TheWorldEndsWithYou'', you'll probably assume from the box art that the two blondes with blue eyes and similar clothing styles are siblings. They even show up together; it's actually surprising when Rhyme mentions she actually just met Beat a few days ago. [[spoiler: They ''are'' siblings, but she's lost that memory.]] The matter doesn't come up until near the end of the game, and ''Neku'' is surprised at the reveal... but Beat is surprised Neku ''hadn't'' figured it out.
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[[folder:Webcomics]]
* The webcomic ''ButImACatPerson'' is all in greyscale except for green spot-coloring...and the eyes and hair (or, in Miranda's case, [[HairDecorations hair decs]]) of certain characters, mostly Beings and their Masters. If you see someone with colorful hair, odds are, they're going to be important.
* Happens several times in ''{{Drowtales}}'', to the point that fans have learned to look out for this trope when new characters are introduced, despite (or perhaps because) of the fact that most drow actually dye their hair from its natural white.
** Possibly invoked by Kalki, who has light purple hair that is actually between the shades of her biological parents, Mel'arnach Val'Sarghress and Snadhya'rune Vel'Sharen. The latter was something that fans had already [[IKnewIt guessed]] due to this trope and a ShrugOfGod that more or less confirmed it, but the former might be where this trope is invoked since it's implied that her hair was white originally and was dyed, perhaps as a tribute to the other half of her lineage.
** There's also an extremely subtle one that you'd probably only notice if you went an analyzed the colors yourself (and was first pointed out via WordOfGod) involving the same family: Ariel's natural hair color looks white, but it's actually an extremely light shade of purple, in fact a lighter shade of her sister Mel'arnach's. And it turns out Mel is actually her biological mother, not her sister as she's been lead to believe.
** Double subverted later, crossed with a case of RedHerring. When [[http://www.drowtales.com/mainarchive.php?sid=9631 a dark elf with a dark purple hair]] was spotted in one scene, fans immediately noticed the identical hair color to Diva'ratrika, and assumed the elf was another BodyDouble or golem made to resemble the deceased Empress, with only a few people noticing that the arms were unusually muscular for a female. Turns out that it's actually Zhor, a character who [[ItMakesSenseInContext had looked like a spider until now in a new body]], but he still counts for this trope since he's actually the father of the aforementioned Ariel, meaning his hair color was a spoiler for a relation, but to the ''wrong'' character than first assumed.
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