[Please edit changes into the main draft.]
If a character has an element of duality to their nature, such as a Superpowered Evil Side, mixed-species ancestry, a Split Personality, or even something as simple as a Tsundere's two distinct moods, this duality can be foreshadowed or visually reinforced by introducing a dual nature to their visual design. Whether it's two-tone clothes, mismatched eyes, or hair that changes color at the part, the two sides of their character come out in their appearance.
This aesthetic link can be a rule of the universe, such as all werewolves having a brown eye and a blue eye, or a fortuitous in-universe coincidence. Not every case of duality in appearance is part of a greater motif for the character--and would thus not be this trope--but if attention is drawn to both the outward and inward sides, particularly if by another character, then there's little doubt that the former is there to draw emphasis to the latter.
A Duality Motif is inherently limited to single characters, with many other tropes covering visual contrast between characters. The one exception is that when two people from form The Dividual--including Single-Minded Twins--the contrast between the two bodies can highlight contrast in the shared personality.
See Peek-a-Bangs and Skunk Stripe for visual designs that can be--but are not always--used for this effect.
Nei in Avenger has heterochromia to indicate that she (or he, it ain't exactly clear, either) is both a Robot Girl and a real human. Well, actually, she/he is neither but has the potential to become one of the two.
The Count from Gankutsuou has a red eye and a green eye. This symbolizes not only his moral ambiguity and inner conflicts, but also the fact that he is part-human, part-monster, part-alive, part-undead and part-earthling, part-alien through his symbiosis with Gankutsuou. The duality symbolism is also present in some representations of his crest, a stylized heart divided into two clear compartments with different colours to represent his remaining human feelings and the fact that he is gradually turning into a cold-hearted monster.
In Magical Girl Lyrical Nanoha ViVid (which as the name implies, stars Vivio as the main character), Einhart Stratos also has heterochromia. Like Vivio, she has an adult form, based on her ancestry to another Belkan Ruler, Hegemon Ingvalt.
The eponymous Hellhound of Nora; when in human guise, his right eye is red and his left is yellow.
The Count D from Pet Shop of Horrors has heterochromia, one eye being purple and the other, whom he hides under his hair, is golden. In the manga, it's explained that this is meant to be representative not only for his non-human nature, but also for his inheritance: his father and grandfather have golden and purple eyes, respectively. Since each one has a very different personality, and D has personality traits from both, his heterochromia symbolizes this dual nature.
In Sukisho, whenever Yoru takes over Sora's body, one of his eyes turns gold while the other remains blue.
Lord Darcia of Wolf's Rain has one blue eye and one golden wolf's eye, to show that he's found a way to turn himself into a wolf so that he can enter the Paradise forbidden to humans. Fortunately his plan is thwarted by Kiba and Cheza, but his evil wolf eye lives on...
Yu-Gi-Oh! GX: Yubel has one orange eye and one green eye. In Season 4, after Judai merges with Yubel, his normally brown eyes change to these when using his newly acquired Green Lantern Ring powers.
Duo Damsel in Legion of Super Heroes can split into two people, and in some versions has two eyes of different colors. This is strange since 1) she had the ability to split into three before one body was killed, but didn't have three eyes, 2) it's a planetary power and other people from the planet don't have this, and 3) when she splits, each body has two of the same color eyes, making it impossible for her to pretend to not be split when she is.
[[folder:Film -- Live-Action]]
In The Unborn, the 2009 film, one of Casey's eyes starts changing color to match that of her unborn twin brother around the time his soul--which is attached to hers--starts killing people.
The Shining uses a lot of this, with mirror reflections and symmetrical items such as double-doors, to reinforce the dual-nature of the hotel (which is practically a character itself) as well as Jack Torrance with his inner demons.
In District 9 the iris of Wikus's left eye turns gold (and huge) at around the seventy-second hour of his transformation into a Prawn. By the end of the movie, his eyes match again due to his complete transformation.
In Dont Look Back, the characters' appearances slowly change over the course of the movie. Characters who are midway through their transformations often have mismatched eyes.
In the Roger Zelazny book This Immortal, protagonist narrator Conrad Nomikos has Mismatched Eyes, among other odd features. But then he is probably a mutant and is definitely hundreds of years old. His unique appearance is an important plot point.
I glare at people through the cold blue one on the right side when I want to intimidate them; the brown one is for Glances Sincere and Honest.
In Jostein Gaarder's philosophical novel Maya, the character Laura has one brown eye and one green eye. The narrator thinks of this as somehow connected to her dual nature - one side of her seems to care for all living things and the planet as a whole, while the other side seems to care about everything in the world except human beings.
In Stardust, the protagonist Tristran's mismatched ears (one ordinary, one pointed) indicate his mixed human and magical parentage and resulting dual nature, which the reader knows about but Tristran does not.
In an episode of Get Smart Max visits a circus which has a half-man/half-woman, who dresses half in a tuxedo and half in a Pimped-Out Dress. He/She moves his/her head to profile whichever of his/her two halves are speaking at a given time, and occasionally argues with him/herself.
Lieutenant Aiden Ford of Stargate Atlantis has one eye turn completely black due to an accident during a Wraith attack on the city. This same accident also got him hopped up on and addicted to Wraith enzyme, causing him to go AWOL and hunt down the Wraith on his own.
In the WKRP in Cincinnati episode "Dr. Fever and Mr. Tide" Johnny Fever gets a job as a TV DJ spinning disco records. He doesn't want to do disco so he invents an alternate persona named "Rip Tide" for the TV gig. He dyes his blond hair dark for the part, but only the part of his hair not covered by his beret. This duality of hair color reflects the duality of "Johnny" and "Rip," who starts to take him over a la Jekyll & Hyde.
The Drawn to Life games has Heather. a little girl whose face is half covered in shadows. She is Mike's sister in the real world
Yuna from Final Fantasy X has heterochromia to indicate that she's half Al-Bhed. She has a green eye and a blue one.
The vampire sisters in Castlevania: Portrait Of Ruin have heterochromia to indicate they aren't full vampires yet. Each of them has one eye of the other's normal colour, creating a creepy impression that they've switched eyes.
Levi Tolar, on-again off-again Dragon, originally from Super Robot Wars Alpha, has one red and one golden eye, to match her mecha Judecca. Her eye color changes fairly regularly though, depending on game and whether her Levi personality or her Mai Kobayashi personality is dominant.
Ragna the Bloodedge of BlazBlue has these to indicate that he is half-Vampire. However, this only applies in one particular instance of the Stable Time Loop: in all others, including the one covered by the story, his changed eye, the red one, is linked to the function of his arm, the fake Azure Grimoire. When it stops working, so does his arm.
In Phoenix Wright: Ace Attorney DS, Angel Starr has differently colored eyes, but it's hard to tell because the eyes are tiny and we only see one eye at a time. (When the hair covers the right side of her face, she looks nice, but when she's angry it covers the left, so we never see them both at once.) According to the creators, it's because she wears colored contacts, and is thus Invoked
Most of the Val'Sullisin'rune family in Drowtales have mismatched eyes, a fact that is commonly known in the setting and referred to as "devil eyes" in the case of the Ill'haress Ash'waren. It is not clear that they have any sort of "dual nature" aside from the case Ash's daughter Faen: the dual personality Blue = Innocent / Purple = Mysterious or Badassfits.
Kade from Eerie Cuties has a yellow/gold left eye and a blue right eye. Of course, he is a Werecat...
The chimera boy Archie from Ellen's second life dreams in El Goonish Shive, although he hasn't been drawn in colour yet, appears to have heterochromia, given that one of his eyes is filled, and the other has a pupil and a gap between the pupil and the outline of the iris.
Sudoku (Jigsaw's sister) in Last Res0rt also has heterochromia, with no special explanation attached other than her father (Cypher) having green eyes and her mother (Meridian) having blue. She's the only one in the family with this trait though.
Berri in Fiyora Nya has one red eye and one green eye. Though they apparently change colors due to her other souls.
[[folder: Web Original]]
In Survival of the Fittest, Dominica Shapiro , who was dual in nature (displaying both a thoroughly villainous and surprisingly heroic side), has mismatched eyes.
In Batman: The Animated Series, as in many of the other works in which he appears, half of Two-Face's face has been scarred by acid, his hair changes color at the mid-line of his head, and his eye on the scarred side has been stained yellow. In this particular incarnation of the character, he wears two-tone black and white suits that further have each clothing element switch color along the mid-line of his body (i.e. Black jacket, white shirt, black tie switches to white jacket, black shirt, white tie.) He is torn between the noble personality of Harvey Dent and the violent 'Big Bad Harv', settles choices with a coin flip, and is obsessed with duality, taking the trope to its logical extreme.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.