Power Dyes Your Hair

You've just got a power boost. The reasons may vary, whether it's from going into Super Mode or your Super-Powered Evil Side deciding to wake up. Maybe you finally got your hands on that MacGuffin you've been after, or the Sealed Evil in a Can decided not to kill you after all. The point is, you're now capable of destroying the world three times over, and you can feel it.

The first thing your friend says? "Dude, nice dye job!"

Power Dyes Your Hair is Exactly What It Says on the Tin: a Power Makeover that applies exclusively or primarily to hair. The degree in which this happens can vary; on the one hand, a character may only have their hair change a few shades, especially if their hair's already light to begin with. Other times, a character may go straight from having Shiny Midnight Black hair to Hair of Gold or worse. Often, this is a good way for a character to maintain a Secret Identity, as the change isn't permanent. Bonus Points if Power Makes Your Hair Grow. May also result from Paint It Black, if the power surge isn't a good thing for the heroes.

Sometimes when the character's hair goes blond, the Curtains Match the Window in the transformed state, with the character gaining Eyes of Gold - signifying the character's supernatural powers - along with the hair lightening.

Compare Locked Into Strangeness, which is what happens when the color change is permanent. A subtrope of Kaleidoscope Hair, when the color change can be for any reason. An Anime Hair trope, obviously.

Examples of Power Dyes Your Hair include:

Anime and Manga

Dragonball Z has the Super Saiyans, the Trope Codifier. A Super Saiyan's hair turns bright gold and his eyes turn green. At one point, Chi Chi lampshades this when she sees her son in this form for the first time and starts berating him for dying his hair.
In Darker than Black: Gemini of the Meteor, Izanagi, sort of formerly known as Yin, has fluorescent blue hair where once she had more standard bluish-white hair, because she is now an Apocalypse Maiden.
In Tenchi Muyo!, when Kagato starts draining Ryoko's energy, his hair floofs with power.
Inuyasha's hair turns black when he turns human. Of course, the disappearance of his doggy ears is a much bigger marker.
In Busou Renkin Black Kakugane makes its user's hair green and dyed user's skin dark and makes its user a Walking Wasteland The most notable are Victor and Kazuki
Mahou Sensei Negima:
- Negi Springfield has his hair turn white during his Magia Erebea.
- Mana Tatsumiya has her hair turn white when releasing her full power.
In Sands of Destruction, Kyrie's hair goes from blond to red when he regains his memories of being the Destruct Code and goes into super-awesome-killer mode.
- And, of course, it goes back to his normal blond afterward.
Lyrical Nanoha:
- The mages, when they fuse with either Rein or Agito.
- Tohma of Nanoha Force, whose hair becomes white when accessing his Black Knight form.
In Shamanic Princess, when Tiara goes into her demon form, her hair goes from fiery red to blood-red.
More subtle example—in Yu-Gi-Oh!, when Yugi switches to Yami Yugi, lightning bolts of blonde hair extend upwards from his bangs that aren't there as regular Yugi.
In Rosario + Vampire, Moka gains silver hair when she takes her rosario off and gains her true vampire power.
Tokyo Mew Mew:
- In the anime version, Ichigo's Transformation Sequence, in addition to turning her into a Catgirl and giving her nice Magical Girl clothes, changes her hair from dark red to pink, and the rest of the girls also have their hair colors changed to brighter shades.
- Another example, present in both the anime and the manga, is dark-haired Aoyama Masaya transforming into the blonde Mysterious Protector Blue Knight, which is slightly odd, because as the Big Bad Deep Blue, he has black hair again; though this one has an excuse of being a part of the Red Herring designed to conceal the character's identity from the audience, not just from the characters.
In Ah! My Goddess, the goddess Belldandy wears special earrings to contain her power. When she takes them off, her hair goes from blonde to silver.
In Bleach, Ichigo's hair turns black when he activates the Final Getsuga.
- When Ginjo goes Bankai, his hair goes from sleek black to pale white
The Slayers: Lina's Giga Slave leaves her red hair a platinum blonde so light it's almost silver for a day or so after she casts it. Not to mention that time it glowed golden because the spell involved channeling the Lord of Nightmares, who replaced her soul when it backfired.
This tends to happen in the Pretty Cure franchise.
- In addition to getting absurd hairstyles, Pretty Cures' hair get a lot brighter in color—a great example is in Fresh Pretty Cure.
- This, of course, is averted in the first series, when the eye color of the main characters change and it's implied that anyone can recognize them if they take a good look. Of course, you can count on one hand the number of times where they battled in the presence of someone, since everytime the villains appear, everyone conveniently faints, and the few times someone awaken in the middle of the fight, they never get to see their faces.
Variation: the wings of "Charcoal Feathers" in Haibane Renmei slowly turn from light charcoal grey to inky black if they become "sin bound" by doing poorly in their Redemption Quest.
In Mushishi, one Victim of the Week's hair is green because she's merging with a mobile, living swamp, though it turns back to normal once She Gets Better. Also, the protagonist has white hair and a Glass Eye, just like his Obi Wan, as a result of his Origin Story.
In Monochrome Factor, whenever Akira becomes a Shin, his hair turns black.
Akane in Kämpfer turns into a Fiery Redhead in her Kampfer form, Shizuku's normally black hair turns white on the underside and Mikoto turns into a White-Haired Pretty Girl.
In Princess Resurrection, Hiro's hair turns white whenever he senses Hime in danger.
Normally raven-haired Hyoka Kazakiri in "FUSE=Kazakiri" form, during the World War III arc in A Certain Magical Index III, gets Eyes of Gold along with her Hair of Gold... with all of the "good" connotations that both tropes imply.

Comic Books

In Marvel Comics Generic Comic Book, The Hero's hair turned white when he got his superpowers.
Doctor Strange briefly went blond after absorbing the power of one of Shuma-Gorath's generals. He had such an excess of power that he not only obliterated Shuma, he had to 'decompress' through a dozen dimensions, shedding power as he went, just to reach the level where he could safely return to Earth without destroying it.

Fan Works

Amongst the various other effects of becoming a Succubus in The Return is an instant dye job on the hair.

Film

Disney's Tangled has an interesting variation, where the main character was born with her hair dyed by power of the sun. When cut, her hair loses magic powers and returns to its natural brown color.

Literature

In Mercedes Lackey's world of Velgarth, sufficient magic use will not only bleach a mage's hair prematurely white, it will also turn his eyes blue. This is why the Companions are gleaming white and have blue eyes.
In the eponymous Dragonrealm, created by Richard Knaak, people who have latent or active magical power will also have a wide silver streak in their hair corresponding to their level of ability. The streak grows larger as the person actually taps into their power. (The only given exception to this being Cade, whose hair color, for unknown reasons, fluctuates so that on any given day his hair might be all silver or it might be almost completely brown.)

Live-Action TV

Pretty Guardian Sailor Moon: the characters all have normal hair in civilian ID, and get brightly-colored hair during their Transformation Sequences.
On Buffy the Vampire Slayer, after Tara dies, Willow's normally red hair turns black, as do her eyes. Conversely, when she activates all the slayers, her hair briefly goes white. Basically, she's Color Coded for Your Convenience to indicate if she's casting good or evil magic.
The BBC's Jekyll has this as a fairly major physical difference between the meek Dr Jackman and the psychopathic superpowered Mr Hyde—Hyde's hairline is different, but is also much darker.

Video Games

Sonic the Hedgehog has his Super Form; after collecting the seven Chaos Emeralds, he turns gold and is basically invincible. Knuckles similarly goes from red to pink.
- This is taken Up to Eleven in Sonic 3 and Knuckles with Hyper Sonic. Sonic's fur changes colors 14 times a second in this form, following the pattern of white, green, white, orange, white, gold, white...7th emerald color, white, green.
- In at least one source (Sonic CD), it is stated that Sonic's trademark blue quills became that colour the first time he ran at sonic speeds.
Disgaea 2: Cursed Memories does this with Rozalin, when she reverts back to her Overlord Zenon state.
In Dragon Quest VIII, when the main character builds his tension high enough, his hair becomes lighter and spiky. Not that strange considering that Akira Toriyama did the character designs.
In the original Last Blade, Kaede can awaken his inner powers and become more powerful, faster, and blond. He keeps that guise for the second installment of the series.
The King of Fighters: when the Orochi blood of Leona Heidern takes control of her, she awakens her Super-Powered Evil Side and her hair changes from blue to red.
- A non-Orochi example is Kula Diamond, whose hair changes from a chestnut color to light blue when using her ice powers.
In Xenogears, Fei Wong's hair is dark but turns red when he becomes Id.
In Breath of Fire III, turning into a Kaiser form causes the main character's hair to turn gold and sparkle.
In Rayman 3: Hoodlum Havoc, getting different power ups changes Rayman's hair to the same colour as that power up.
In Final Fantasy IX, the Trance System has this effect on every character whose hair we can see. The heroes turn into a white-silver color despite their default. The main villain's hair turns from silver to blood-red.
Both Geo and Sonia in Mega Man Star Force change hair colour when they undergo EM Wave Change. The reason why is never explained.
Inverted The Fancy Pants Adventures: World 3. The initial antagonist is a pirate with a rainbow-colored beard, which causes him ridicule and a coup from his ship. When the player fetches him hair dye of Norris, the pirate restores his men's support and reclaims his ship from the ninjas that invaded in his absence.

Visual Novels

Tsukihime: Tohno Akiha's hair changes to a bright red when her Super-Powered Evil Side comes out.

Web Comics

Played with in El Goonish Shive, when losing her magical abilities turns Nanase's hair black. And then played (relatively) straight when Susan's magical awakening turns Susan's hair dark blue...in large part because that was the color that she was already dyeing it.
In Mind Mistress, Lorelei's blonde hair turns black when she turns into the titular hero.

Western Animation

In Danny Phantom, when Danny first gets his powers and, subsequently, whenever he "goes ghost", his hair turns white. In "Phantom Planet," when he decides to reverse the accident that gave him his ghost powers and get rid of them, he gets a streak of white in his hair, which goes away when he gets his powers back. It is even commented on in-universe.
On Avatar: The Last Airbender, Yue was born with dark hair like most people in the Water Tribes, but it turned white when she took in some of the Moon Spirit's life force.