In 3×3 Eyes: after being coerced and tricked into serving the bad guys, Yoko forgoes her casual clothing for and all-black ensemble with long-sleeved shirt and pants, black encompassing cape and a live serpent draped around her shoulders.
This trope sort of shows up in the Ah! My Goddess! movie, when Keiichi is possessed by Belldandy's old mentor, Celestine. However, although the resultant flowing robes and markings are cooler, he also looks...significantly more righteous and 'good', with the evil only actually being visually hinted at by possessed-Keichi's facial expressions at times.
Shows up better in the Lord of Terror arc; whenever the LoTBody Surfs to someone else, they get an Evil Costume Switch. Egregious in Keichi's case because the evil costume literally materializes out of thin air.
Not only that, he even grows his hair out and dyes it blond in a matter of seconds! The latter of which (dying it blond) only appears in the manga, though. In the anime, his hair is still black when it grows, but unlike the manga, mysteriously goes back to being short after the Lord of Terror is defeated and removed from Keiichi, although the costume still remains on Keiichi (cue his shocked remark at it in the manga).
The Angel Eater arc is also a great example: Belldandy, while implanted with the devil by Hild, gains a tight and revealing outfit and demon wings.
A temporary example in Assassination Classroom. Following Chapter 128, in which she attempts to kill Koro-sensei with her tentacles and reveals her true identity to the rest of her class, Kaede Kayano/Akari Yukimura switches to a new look where she lets her hair down and wears a black dress and heels as well as a red scarf. Once her tentacles are removed, she reverts back to her previous appearance as part of Becoming the Mask.
When J Sawaragi becomes a member of Numbers Elite, he changes to an identical version of his outfit in black.
Though an antagonist from the start, when Number Nine is revealed as the true Big Bad, he also changes the color of his outfit and nothing else. This time, he wears red.
In Berserk, upon making his Face–Heel Turn by making a Deal with the Devil to become the fifth member of the Godhand, Femto, Griffith's outfit is a darker, more sinister, and skin-tight version of the warrior armor that he wore with the original Band of the Hawk. Then he decides to get what he wants and uses less obvious clothing, reminiscent of his old armor while becoming a Villain with Good Publicity.
After his Face–Heel Turn, Tousen in Bleach eventually changes his outfit by throwing out all black elements: while initially he keeps his shihakusho and drops his haori with a white sleeveless coat, he later dons with a sleeveless tunic, and switches his white boots with the usual Arrancar black tabi footwear. He also changes his goggles and orange scarf for a visor and an orange rope.
Played with Aizen (Evil All Along) and Ichimaru (The Mole): the former dons a white longcoat coat over his Shihakusho, but eventually switches the black robes with a white Arrancar-style outfit plus a pink sash; and the latter dons a white long-sleeved robe first over his Shihakusho, and later over his own Arrancar clothing.
Inverted with main protagonist Ichigo, whose default outfit is predominantly black while his inner hollow has a white version of his clothes and sword.
When Uryuu joins the Vandenreich, he gets their classic uniform. That has almost no difference to his old costume theme. Now he wears a Badass Long Robe and looks like a soldier. It symbolizes that he has lost his individuality.
In Fullmetal Alchemist, Ling Yao switches his brightly colored clothes out for an all-black ensemble with a Badass Longcoat upon accepting a Philosopher's stone from Father and becoming Greed. It's later played with: while he sticks with this ensemble for the remainder of the series, it kind of loses the evil part after Greed pulls a Heel–Face Turn.
During the Kodoku arc of Fushigi Yuugi, Tamahome wears a sophisticated black suit to match his nun-chucks, complete with a blue bandanna and belt to replace his red belt, as well as a red earring that, apparently, helped with brainwashing.
In the Galaxy Angel Beta manga based on Galaxy Angel Moonlit Lovers, instead of simply being mistaken for evil as she was in the game, Chitose actually goes evil, changing not only her costume but the paint job on her ship.
Male (insert joke here) example from Get Backers: when Kazuki is Brainwashed and Crazy, he trades his oversized androgynous t-shirts for a form-fitting black tank top with a cross-like design. Amusingly, his shirt starts out tucked into his jeans but ends up baring his midriff by the time the final battle rolls around. Also averted later on: when Juubei, Toshiki, and Sakura are Brainwashed and Crazy, they are given white uniforms—although Sakura's does show her cleavage.
In K: Return of Kings, when Saruhiko Fushimi leaves his Clan and joins Jungle, he switches from wearing his uniform to wearing a mostly black ensemble with a black fur trim on his hood (that echoes the crow-feather look of his hair), and a beaded necklace that looks rather sexy with his V-neck t-shirt. Pair with punky monochrome-British-flag design sneakers and a fizzle of green electricity from his new powers, and we have a surprisingly good example of this.
Inverted. The villains are more covered and wear white, while the heroes are Stripperiffic in black. As a result, a Heel–Face Turn results in former villains stripping down into black, while a Face–Heel Turn results in former heroes covering up in white. When Brainwashed and Crazy, Ryuko dons the white Junketsu and looks far more modest than when she wears her normal Stripperiffic Senketsu. When Satsuki and the Elite Four join forces with Nudist Beach, they go from wearing modest white uniforms to strategically-placed black belts and little else.
Played straight with Mako: she normally wears a white and blue Sailor Fuku, but during episode 7, she changes to a black and red Badass Longcoat, nice hat, and geta sandals when she becomes the Fight Club President (though she still wears her Sailor Fuku). It doesn't last long, though.
When Roll is infected with a virus in Mega Man Battle Network to turn her evil, she gains fangs, a whip, and six-inch heels, in addition to becoming more...shapely. Um...she's, like, twelve...
Mermaid Melody Pichi Pichi Pitch had Sara, who, besides exchanging her orange frilled dress for a black diva outfit (in the manga; in the anime, she just wore her "civilian" clothes all the time), also changed her hair colour. This form is the first form in which we see her.
When Shizuru had her Face–Heel Turn late in My-HiME, she stopped wearing her standard-issue school uniform for a short while and swapped it out for a dark purple kimono. She is seen wearing it again right before her last fight with Natsuki, where she's back to her uniform. When she and Natsuki are revived after their Murder-Suicide, both are in their school uniforms.
Tomoe gets this treatment with her old Robe replaced with a suit of black plate armor.
Nina's Transformation Sequence after her Face–Heel Turn gave her a black silhouette instead of a blue one. There were a few times when she didn't activate it on her own. Other than that, she wore basically the same uniform as before, only darker.
In Naruto, when Sasuke is inspired to do a Face–Heel Turn, he is wearing a different, black one-piece suit. This is compounded after the time skip when he starts dressing like a gaypirate.
Neon Genesis Evangelion contains a subtle male example. After Yui "dies," Gendo switches from his white lab coat and '80s-style prescription glasses to a black version of the typical NERV uniform and orange sunglasses. Even more subtly, Gendo stops shaving and doesn't zip up the front of his military uniform, showing that he more or less stopped caring what he looked like after losing his wife.
Rei usually wears a white plugsuit, but in Rebuild of Evangelion, Seele's new Rei clone wears a black one. Shinji also trades in his classic blue and white plugsuit for a black variant after betraying Misato and siding with his father.
Evangelion Unit 13 starts off with purple and green coloring, but the color scheme changes to white and red during Instrumentality.
Also in the Rebuild movies, in a case of Eviler Costume Switch, when Gendo pulls the rug out from Seele and starts manipulating the Instrumentality and fall of mankind himself, he ditches his trademark orange sunglasses for a crimson-red version of Chairman Keel's ominous, futuristic visor.
Yet again from the Rebuild films, Misato's colder, more distant (and Gendo-like) demeanor is accompanied by a commander's hat, sunglasses, shiny black boots, and a more militaristic version of her red coat (which she now actually buttons up).
The anime version of Prétear had Sasame changing his simple outfit for an incredibly elaborate black and purple one, complete with a demon larva logo, after he joined the Princess of Disaster. Both of them revert to normal at the end, though.
In HeartCatch Pretty Cure!, all Desert Apostles except Dune and Dark Precure got a new costume when they joined... the Desert Apostles.
In Suite Pretty Cure ♪, every male Minor Land villain got one when they were brainwashed. And all of them plus Siren, who didn't get one, get a Good Costume Switch after being de-brainwashed.
In Princess Tutu, when Mytho's heart is poisoned with Raven's blood, he gains a new costume that is all black with raven feathers. Rue's outfit as Princess Krahe might also count, although it is more a case of Dark Magical Girl.
In Queen's Blade Rebellion, several members of the old cast get costume switches, but Elina's and Ymir's especially reflect their change from being on the side of the "good guys" to being on the side of the "bad guys". Goodbye tiger-striped amazoness costume with blue bra-armor for Elina, goodbye pink and white doll dress for Ymir, hello black leather with metal protrudings and red and grey off colors. Claudette also switches out her amazon costume in favor of a regal white getup.
In the original anime, Listy gets this in a big way when she is brainwashed by Aldra. She goes from a revealing but still rough-around-the-edges brown leather outfit with a tousled hairdo to a silky black jumpsuit sporting Navel-Deep Neckline and slick, straightened hair worn long. Aldra herself goes through this in reverse between the first series and rebellion, going from an outfit that's all intimidating sharp edges to the garb of a housewife.
Rage of Bahamut: Genesis has Jeanne after she is left to burn at the stake gaining a darker/more revealing armor out.
Tuxedo Mask's much cooler "Prince Endymion" armor is first seen out of flashback when he is captured and brainwashed by the Dark Kingdom.
Sailor Mercury gets a costume upgrade when she becomes Dark Mercury in Pretty Guardian Sailor Moon. Her Transformation Sequence also improves, although that's more likely a case of the special effects crew honing their skills as the series progressed.
Little Chibi-Usa/Rini, whose face-heel turn involved transforming from a cute little girl into a twenty-something seductress in a slinky black dress. Apparently, evil isn't only cooler; it's also sexier. More, she even tries to seduce her own father in the manga. In the anime, she hates him and her mom and tries to kill them both. The enemy still tries to brainwash her mom from the future with similar imagery, but they fail.
In Saint Seiya, in the Asgard saga, when Polaris Hilda is a good person, she wears a light blue dress. When she is possessed by the Nibelungen Ring and forced to do evil, she starts wearing a crimson and black colored war dress with a black cape. After she's freed from the ring, she returns to her light blue dress.
Senki Zesshō Symphogear: Miku gets unwillingly subjected to this trope twice. In G, she gets captured and experimented upon by Dr. Ver, who has her bonded to the relic Shenshoujing - which has a distinct horned mask as part of its Armed Gear - and manipulates her into Fighting Your Friend. Then in XV, she gets captured by Noble Red and turned into the vessel for Shem-ha, who alternates between a black bodysuit and a combination between a Faust Robe and the aforementioned Shenshoujing and in both costumes is seen wearing a hi-tech version of a Mesopotamian horned cap. After being freed from Shem-ha's possession, she joins the cast as an 11th-Hour Ranger while wearing the purified Shenshoujing.
Shakugan no Shana, Snake of the Festival Sakai Yuuji. He even gets a new hairdo aside from black armor.
Katsumi Liqueur of Silent Möbius obtains a black cloak and tight leather clothes when possessed by the demonic sword Medium.
A villain from the first season suffered a full-body burn at the hands of Lina Inverse. He never bothers to remove the bandages (which make him look like a Mummy) until after his Heel–Face Turn.
What's more, an evil priestess who wears a full-on black-leather-with-cleavage-and-spikes ensemble is shown in flashbacks wearing a white robe. Why she made the change, since she never admits to being evil, is uncertain.
Justin Law in Soul Eater. The crosses on his priest's robes get replaced by the three eyes of the Kishin. Kid gets a minor version of this following his...encounter in the Book of Eibon: his black and white colour scheme stays, but his suit's all black except for white stripes and a cravat (?).
When Cellaria in Soul Linkreveals her true intentions as The Mole, she spends the rest of the series wearing a black, very stripperiffic costume that stands in sharp contrast to the green military uniform she was wearing before. She also lets her hair down and crushes her glasses, apparently taking a page from the book of Bleach's Aizen.
In Tales of Wedding Rings, when Krystal falls under the sway of the Abyss King and undergoes a temporary Face–Heel Turn, she trades out her normal attire for a very skimpy black outfit, and her weapon likewise turns black and becomes spikier.
In episodes 21-23 of Tiger & Bunny, we see that both the fake Wild Tiger and Barnaby (though, in his case, it's symbolic of antagonism towards Kotetsu, not 'evil') are sporting black versions of their suits. Barnaby goes back to his usual suit halfway through ep. 23, after regaining his memories and siding with Kotetsu once more.
Kaneki undergoes an Anti-Hero version of this trope, changing from a mousey young man in either baggy clothing or a waiter's uniform to favoring a sleek, black battle suit with cutouts. Even in casual clothing, his style has shifted to favoring monochrome, flattering looks.
In the sequel, :Re, Seidou Takizawa's transformation has seen him go from dark-haired in conservative suits to white-haired with black lips and nails, and creepy black robes with bare feet.
Transformers: Armada features Wheeljack (no relation to the more famous Transformer with the same name). Originally a white Autobot, he performed a Face–Heel Turn and joined the Decepticons, and the next time he was seen, he'd started "wearing" a black sports car as his alternate mode.
X/1999 has this when Subaru makes his Face–Heel Turn and switches his white coat for black.
During the Yu-Gi-Oh! Battle City story arc, immediately after Malik's subconscious dark side takes over and he becomes even more villainous than before, in the very next scene, only minutes later, he has traded his lilac vest and black pants for a black shirt, beige pants and a purple cape.
Played with Yugi and his good Superpowered Evil Side. Sometimes, Yami Yugi throws his jacket over his shoulders and wears it like a Coat Cape, demonstrating that "Evil Is Cool".
In the second season of Yu-Gi-Oh! GX, the bad guy's team's color is white because of the religious overtones of his cause of destroying the world and rebuilding paradise, and all the brainwashed members of his cult wear white. In the episode when his first convert returns to normal, Jun swaps the white trenchcoat for his traditional black one — and his normal outfit underneath inexplicably reappears along with it. In addition, Kaiser Ryo dons a black trenchcoat after his Freak Out and Judai wears a black full-body armor after being taken over by his Superpowered Evil Side. And under Yubel's possession, Johan wears a sleeveless outfit that otherwise resembles bondage gear, cranking up the Ho Yay with Judai to even more ridiculous levels.
A popular trend in Yu-Gi-Oh! 5Ds, as performed by both Carly, after coming back after being killed by being thrown off a building, and Misty, wanting revenge against Aki/Akiza for killing her brother, each gaining new black outfits, black colored eyes, and facial markings.
Sherry gets a new black and white outfit after she becomes allies with Zone.
III Arclight from Yu-Gi-Oh! ZeXal. During Episode 47, the middle of a mini-arc and his duel with Yuma Tsukumo, he gains gladiator-type armor, complete with cape and helmet and everything. Definitely marked that he was getting serious, considering that right after, he took away Yuma's 'kattobingu', essentially making him lose all self-confidence and will to win. Not even mentioning how in Episode 48, he killed Astral, Yuma's best friend. While he did help revive him in Episode 49... It marked a serious change from III's normal personality.
◊ gained a more sinister
