Trish and Don's relationship in Finding Gaia is partly based on the partners' interest in each other's redness/blueness.
The sons of Eärendil, Elrond and Elros in J. R. R. Tolkien 's The Silmarillion. Elros Tar-Minyatur was the Red Oni, Elrond was the Blue Oni. Elros became the first king of the Númenor and High King of Men, while Elrond became the wisest of all Noldor in the Middle-Earth.
"Most Wimseys were like the 16th Duke, and his father: 'Bluff, courageous, physically powerful' but not very intelligent; of hearty and voracious appetites of all kinds. They could be 'cruel, yet without malice or ingenuity.' The other type is physically slighter, smarter, with great nervous energy, and 'lusts no less powerful, but more dangerously controlled to a long-sighted policy.' These became churchmen, statesmen, traitors; but sometimes poets and saints.
Lord Peter, of course, is of the second type.
In Camus' The Stranger, the borderline-apathetic narrator Meursault is contrasted with the violent, passionate Raymond.
The book religious subtext intentionally organizes them as this too, since Peter and Lucy represent the better apostles, Peter being named after Apostle Peter, who was the first Leader of the Christian church after Christ ascended and Lucy representing Saint John, who had the most faith in Jesus, just like Lucy has the most faith in Aslan. On the other side, Edmund was intentionally made by the the very own creator to represent Judas, the iconic betrayer, and Susan represents "Doubting Thomas", because she completely loses her faith in Aslan and Narnia, never reaching Aslan's Country (Heaven) again.
In Harry Potter, the Marauders' group has two sets. James Potter and Sirius Black, represented by Red, are dark-haired, rash, prankish, and Hot-Blooded. Remus Lupin and Peter Pettigrew are calmer, quieter, and more collected than them, having blue traits.
Wizards (dangerous fantasy) and Muggles (relatively safe reality)
Gryffindors (hotblooded) and Ravenclaws (intellectual)
Hufflepuffs (just happy to be here) and Slytherins (ruthlessly ambitious)
Ron (emotional and passionate) and Hermione (intellectual and cool)
It fits racially. Ron is a pureblood, and Hermione is a muggleborn.
They may symbolize alchemical Sulfur and Mercury respectfully.
This carries over in a myriad of ways in George R.R. Martin's A Song of Ice and Fire. In addition to grandscale comparisons like the rapidly shifting seasons and entire royal families like the Targaryeans and the Starks, this dynamic also carries out between several sets of important characters:
Robert Baratheon and Eddard "Ned" Stark.
Jaime and Tyrion Lannister.
Arya and Sansa Stark.
Cersei Lannister and Catelyn Stark.
Oberyn and Doran Martell.
Renly and Stannis Baratheon.
The Big Bad Duumvirate of The Thrawn Trilogy consists of the blue Grand Admiral Thrawn and the red Jedi Master Joruus C'baoth. Thrawn is calculating, Affably Evil, a scarily good planner, appreciates art, respects his men and his Commander Contrarian, extremely pragmatic, and has a temper, but overall is A Lighter Shade of Grey compared to, well, any of the Empire's previous leaders. Also, he has blue skin. Joruus C'baoth is domineering, believes himself to always be entirely in the right, treats non Force-Sensitives like puppets, calls the Empire his Empire, flies into a violent rage at random, constantly rejects any authority but himself, and is insane. Joruus fell to the Dark Side long ago, and the Dark Side is often represented with red. The two are often at odds and Thrawn reliably comes off looking better.
In Outbound Flight, the two highest-ranking people on the Trade Federation taskforce sent by Darth Sidious to destroy Outbound Flight are Kinsman Doriana and Viceroy Siv Kav. Kav is Red - he's impatient, he tries to attack Commander Thrawn's little picket force right away, he resists and blusters when Thrawn captures the two, he never learns to pronounce Thrawn's full name properly and continuously underestimates the commander, and he gets killed when he tries to kill Thrawn. Doriana is Blue and a foil, and not only does he live, he plants the seeds to Thrawn's joining the Empire.
It's easy to tell that Timothy Zahn likes playing the calculating type off the irrational type, and the calculating type tends to win.
Among Ultramarines, Captain Sicarius is Red to Captain Agemann's Blue. While Cato is hot-blooded, prone to Chewing the Scenery and charging straight for the enemy, Severus is slower, likes to think things out and have worked-out tactic before comitting to the fight, not to mention that he's Combat Pragmatist while Cato practices Honor Before Reason.
Crimson Fists have Captain Cortez as Red and Chapter Master Cantor as Blue. Cantor has soft voice and commanding presence and bothers himself with mortal matters such as administration or politics, while Cortez is impatient, disdainful of mortals and fueled by The Power of Hate.
Invoked literally in Monster Hunter Vendetta. The heroes have to defeat a pair of rampaging oni, one stronger and the other more devious, who attack a heavy metal concert.
In the Honor Harrington series, Nimitz is the Red oni to Honor's blue. He's a direct and uncomplicated soul, who is emotionally open (as befits his species), and will obliterate any enemy that comes in reach (particularly doable when you're as formidably armed as he is). Honor is the cooler, less emotionally open (though she's not emotionless) and more intellectual and nuanced member of the pair. She, true to the trope, likes water while Nimitz (being a felinoid) doesn't.
You also get a distinct Red/Blue flavor when Queen Elizabeth and Honor interact.
In The Outsiders, the difference between the greasers and the Socs is described as being that the Socs don't allow themselves to feel emotion, while the greasers feel emotions too violently.
From the realm of Tortall (and the Protector Of The Small series, specifically) come Nealan of Que and Keladry of Mindelan. Neal personifies the Red with his poetry, wild emotions, and inability to suffer any injustice and Kel breathes Blue with her emotionless shell and refusal to let others get to her.
In Death: When there's a pair of murderers, they will likely fall under this trope, like in Seduction In Death and Indulgence In Death. Would it surprise you to know that the Red Oni tends to be the weak spot of the two?
The Jennings series' two main characters, Jennings and Darbishire, follow this dynamic: Jennings is impulsive, brash, and extroverted (red), while Darbishire is calm, timid, and introverted (blue). The two main teachers can also be classified under this trope as Mr. Wilkins is a temperamental and emotional Jerk with a Heart of Gold (red) while admitted Author Avatar Mr. Carter is calm, reasonable, and intellectual (blue).
Sherlock Holmes: Holmes is the cold, calm, cool intelligent Blue Oni to his friend's more emotional Red Oni. However, Watson is the Blue Oni in combat, whereas Holmes is the Red Oni when it comes to cases. When his friend is wounded, he's a mix of both Onis gets in Tranquil Fury mode. It is not a smart idea to send him in to Tranquil Fury mode by threatening his friend.
In Dora Wilk Series, Odd Friendship of Miron and Joshua has one as Red and the other as Blue. Miron usually wants to charge head-on into danger and his magic is much more offensive, while Joshua's magic is based around healing and he prefers negotiating to fighting.
It's possible that a lot of the success of the Twilight series (Team Jacob and Team Edward, anyone?) is due to Stephenie Meyer's playing off two such extreme examples of Red Oni (Jacob) and Blue Oni (Edward) against each other as love interests.
In Les MisÚrables, Enjolras the fiery revolutionary warrior (red—witness his trademark red vest in The Musical) and Combeferre the gentle progressive philosopher (blue) are presented as ideological foils. Personality-wise, however, the coldly chaste Enjolras is the blue oni to the earthy, loquacious Grantaire's red.
Coyote: Carlos "Rigil Kent" Montero (red), Wendy Gunther (blue).
Simona Ahrnstedt does this in Íverenskommelser, dividing different couples into different "Onis". Beatrice and Seth are more rebellious and hot-tempered (red), while Sofia and Johan are more calm and laid-back (blue). Jacques and Vivienne, the second Beta Couple, is yet another hot-tempered red oni pairing.
"De skandalösa" has a similar set-up. Gabriel is an impulsive red oni. And while Magdalena tries to be a composed blue oni, she's really a hot-tempered red oni. Ossian and Beata are their more calm and laid-back blue oni Beta Couple. And we also get Venus, a sweet-natured and docile Proper Lady (blue), and Nora, an emotional Lady Drunk (red).
Deltora Quest has Jasmine as the red to Barda's blue. Lief falls somewhere between the two, usually acting as the blue to Jasmine's extreme red, or red to Barda's 'older and wiser' blue.
You could also argue that the city of Del is the red to the city of Tora's blue.
Elena and Lila in My Brilliant Friend by Elena Ferrante is one such relationship. Elena is the blue (highly educated but a Shrinking Violet) to Lila's red (passionate, left school early). Elena feels inferior to Lila for having more passion to get what she wants while Lila is jealous of Elena's education allowing her to leave their poor hometown.
The Raven Cycle does this somewhat with the four main boys. Ronan is the red to Gansey's blue, but they contrast in that Ronan is more closed off from others while Gansey is more sociable, as well as obsessive, giving them some traits of the other color. Adam tries to be blue in order to fit into high society, but is ultimately red. Finally, calm, quiet is Noah blue, evening out the quartet.
The female protagonist, Blue, ironically plays the Red Oni to her mother's Blue Oni.
She and Adam also serve as Gansey's Red Oni when Ronan's not around.
In Edgar Allen Poe's poem The Raven. The Narrator/The Lonely man becomes the increasingly infuriated Red Oni to the silent and ominous Blue Oni Raven.
The Reynard Cycle: Reynard and Isengrim respectively. Reynard is a thief with no respect for authority, is a borderline pyromaniac with a terrible temper (when unleashed), and favors the color red. Isengrim is from the icy north, is a stoic (being almost supernaturally calm, cool, and collected), and has bright blue eyes.
Red-haired, energetic, and slightly impulsive Rose Brier is the Red to her cold Fish's and black-haired, sensible sister Blanche's Blue Onis.
Rose Hathaway and Lissa Dragomir from Vampire Academy. Rose is passionate, defiant, often violent and impulsive. In contrast to the more obedient, reserved, and intellectual Lissa.
Diana and Billie from The Final Cut — passionate, hyper sexual Diana is the Red Oni to shy, reserved Billie's blue.
loud, bossy director Richard is also the Red to gentle screenwriter Fay
Violent, foulmouthed Claudia could also stand in for Richard.
In The Lost Fleet, the competing advisors Rione and Desjani are this trope. Rione is a calculating, reserved politician with pragmatic motivations; Desjani is a bold military captain who believes their mission is divinely ordained. One of the protagonist's early challenges is mediating between the two.
Swedish crime writer Mons Kallentoft pulls this trick in his series about detective inspector Malin Fors. In Dem Femte Årstiden (The Fifth Season), Malin is the Red to a group of Blue male detectives. Especially when dealing with a sexually sadistic serial killer. The only other Red in her group is Waldemar Ekenberg, a principled thug who often steps over the line when interrogating the sort of perp who really makes him feel sick.
In Lottie and Lisa, loud, bratty and carefree tomboy Luise is the Red to her calm, mature and obedient twin sister Lotte's Blue.
The two main characters from the Star Trek: Department of Temporal Investigations series, Lucsly and Dulmur, are blue and red, respectively. Both lean more intellectual, as required by their job, but Lucsly is stern and humorless and *very* rule-abiding, while Dulmur is more easy-going. True to form, Lucsly has to endure a crisis of faith, complete with a 10-Minute Retirement, when his deep-seated beliefs are called into question.
In the series Gus and Waldo, we have a respective Blue and Red Oni as a Anthropomorphic Penguin Gay Couple.
The Dinosaur Lords has the two Grey Angels shown on-page as a Red (Raguel) and Blue (Uriel) Oni.