An Eye Colour Change is when a character's eyes change colour in response to some stimuli. The colour the eyes change into depends on the event and the character, but it can be anything from a shift in hue to something as dramatic as brown eyes turning bright yellow and round pupils lengthening into slits.
This is generally a sign that something major has just happened. A possession, a shift in alignment, the acquisition of supernatural power, trauma, anything significant can trigger an Eye Colour Change.
Supertrope to Kaleidoscope Eyes which is about eyes working as mood rings. Does not apply to a Adaptation Dye Job. See also Convenient Color Change, Game Face, Eye Lights Out and Demonic Possession.
Examples
Anime and Manga
Claymore: The eyes of a Claymore are silver until they use their Yoma powers; this turns their eyes gold.
Subaru Nakajima's eyes in Magical Girl Lyrical Nanoha Striker S are normally green, but turn gold when she enters full Combat Cyborg mode. The first time this is triggered on screen, it also coincides with Unstoppable Rage.
The title character of Inu Yasha's eyes change colour according to whether he is in human, hanyou, or youkai form.
Dragon Ball Z: A Saiyan's eyes turn green when he turns Super Saiyan (although it's easily missable, since the other effect of the transformation is much more visible).
In Naruto The Sharingan is a hereditary superpower of the Uchiha clan that allows for Awesomeness By Analysis and causes distinctive change in the eye when used; a more advanced version exists has a different look. Both are unlocked by traumatic experiences.
In Tengen Toppa Gurren Lagann, humans who channel enormous Spiral Power have spirals appear in their eyes. This is seen mostly in Lordgenome, but also briefly in Kittan right before his death. It's strangely absent in Simon until the Distant Finale.
The eyes of NEXT in Tiger And Bunny typically change to a bright, glowing blue when they activate their abilities. This isn't the case for all of them, however -- NEXT with passive abilities don't have an eye color change, the eyes of the two NEXT in Ouroboros glow orange-yellow, and Kotetsu's eyes have recently started turning red to go along with his Power Incontinence.
In Yumeria, Neneko has violet/purple eyes, but when mental time traveler Neito takes control of their shared body, they turn green.
In Inu Yasha, both the title character and his half-brother, Sesshoumaru, have eyes that go completely bloodshot-red when their demonic powers are fully released.
Film
In The Lost Boys, vampires' eyes go from their natural colour to red when they transform into 'vampire mode'.
Star Wars has the phenomenon of "Sith eyes", related to the Dark Side of the Force. They usually aren't permanent and usually manifest when the dark Force-user is enraged (Count Dooku, always calm and collected, never manifests them). Only Darth Maul, who is more or less always belligerent, sports permanent Sith eyes.
Literature
Rachel from Cliff McNish's The Doomspell Trilogy has eyes that change colour depending on the spell she is using, these eyes are said to be a wizard trait.
At the end of Septimus Heap - Magyk, Sarah Heap notices that Septimus's eyes are becoming green from exposure to Magyk when she identifies him as her child.
In The Stormlight Archive, it is believed by many characters that if a person with dark eyes wins a Shardblade their eyes will become light. Notably there is one character with a Shardblade who's eyes are normally dark green but become pale blue when he is actively using the Blade, the character notes that this is unique to his particular blade, so it's not entirely clear what the actual situation is.
In The Wheel Of Time, users of the True Power develop black flecks swimming across their eyes as a result of overuse. With continued use, the flecks become more common, and eventually transform the eyes and mouth into caverns of flame.
In The Door Within Trilogy, the Glimpses eyes change colour depending on their affiliation: Blue for King Eliam, Red for Paragor, and Green for neutral/undecided.
Whenever Willow makes intensive use of magic, her eyes abruptly turned pitch-black while closed. Then, in the series finale, when using the scythe to activate all the potential slayers, her eyes (and hair) turned white, hinting she was using powerful white magic.
When Willow buffed up Buffy with magic to fight Adam, Buffy's eyes turned gold/yellow.
In Forever Knight, vampires' eyes turn golden when they vamp out.
In Merlin, Merlin's eyes turn brownish-gold when he uses magic.
In Tales Of Symphonia, Colette's blue eyes turn red when she loses her soul; they return to blue when she gets it back again.
In Tales Of Symphonia Dawn Of The New World, Emil's normally green eyes turn red whenever his "Ratatosk-mode" personality is in control (usually during battle only).
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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.
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