Hair Color Spoiler

A Hair Color Spoiler is a plot twist that is spoiled by bizarre and improbable hair styles often seen in anime. In its most common form, a character is introduced who is related to an existing character. The writers might start dropping hints right away about their relationship, but it doesn't matter -- you already noticed that only those two characters have that exact shade of blue hair -- because You Gotta Have Blue Hair.

Anime and Manga

 * Zero no Tsukaima stretches this out. In season one, we learn about Tabitha's backstory and meet her mother, who has the same exact shade of light blue hair as her. Her butler, Mister Exposition, tells us about Tabitha's evil uncle who usurped the throne. The story is dropped until season three, which introduces the mastermind behind the mastermind behind the mastermind of the events of season one. He wears a crown, and his hair color is the exact same shade of blue as Tabitha's and her mother's.
 * In Yu-Gi-Oh Season 5, it is supposed to be a huge reveal that Leon Wilson is actually the younger brother of season villain Zigfried von Schroeder. Except that the von Schroeder brothers are the only Yu-gi-oh characters ever to have purple hair.
 * In Faster Than a Kiss, Fumino, at one point, meets a woman who, at the end of the chapter, is revealed to be Shouma's mother/Kazuma's stepmother, completely shocking Fumino...even though the woman looked almost exactly like Shouma, even down to the hairstyle.
 * In Pandora Hearts, Jack's identity is kept mysterious for some time...except the series pretty heavily uses similar hair/eye colors for families, and Jack has literally Oz's hair except with a braid attached. Little surprise when it's revealed that Oz is his descendant and partial-reincarnation.
 * Naruto: was guessed by Genre Savvy readers from the moment he first appeared, mostly due to their similar hair and facial structures.
 * Gundam AGE: approximately 75% of the fans knew that Flit and Emily would end up together, mainly because images of Flit's future son, Asem, show him with blonde hair like Emily and not some shade of purple or blue as would be expected if Yurin was the mother.
 * Betterman: The heroine's brother has gone missing. The good guys are repeatedly helped by a mysterious man with the same odd and otherwise unique tri-colored hair shock the heroine has. Yep, it's her brother.
 * In Saint Seiya, people inhabited by a god have hair colors not found in nature. Andromeda Shun has green hair, though this is never remarked upon by anyone. Come the Hades plot arc....
 * Black Butler: Combined with eye colors for . Angela and   are the only character to have white hair and Purple Eyes (though Angela's hair is blueish).

Film

 * Rapunzel's birth parents from Tangled have brown hair, while their daughter has blonde hair due to the queen consuming a magic flower. Guess what color Rapunzel's hair turns into after being cut at the end of the film!

Live Action TV

 * In Pretty Guardian Sailor Moon, a new senshi appears at the end of an episode, sporting blue cat ears, a blue tail, and blue hair. It is 'revealed' in the next episode that she really is Luna, the, you know, blue-furred cat.

Video Games

 * In Chrono Trigger, after traveling to the Kingdom of Zeal in 12,000 BC, you meet Queen Zeal and her children, Schala and Janus. The three of them have the exact same shade of light purple hair which is found on nobody else -- except Magus, who is actually
 * In Legend of Dragoon, the revelation that party-member Meru and penultimate villain Lloyd are both Winglies is spoiled by their platinum (with a blue tint for Meru) hair, which is exclusive to Winglies.
 * In Pokémon Black and White, the relation between N and Ghetsis is played as a very dramatic twist. However, more Genre Savvy fans were speculating some sort of relationship before we even knew much of who they were, starting the moment they noticed such close shades of green and distinctive curls/fray/whatever-you-want-to-consider-it.
 * Tales of Destiny: The only party member with hair similar to Rutee's (dark, cut short) is Leon.
 * Notably avoided in Last Scenario. A One Degree of Separation twist is hidden by the fact that one character's hair changed color, and by the time that fact is revealed, the player probably won't think to connect them to a random person in someone else's backstory.
 * In Golden Sun games, only Adepts ever have Anime Hair in wacky colors and styles, usually reflecting their elemental affinity. Knowing this, anybody picking up Golden Sun Dark Dawn will immediately identify Amiti (cyan-blue) as a Mercury Adept, and.
 * Amiti's blue hair is a twofer, since he lives in an area otherwise exclusive to non-Adepts with brown or black hair. This gives away the fairly early reveal that he's not entirely of local stock.
 * There's an entire subplot about people noticing that Amiti kinda looks like that one other plot-important bishounen with cyan-blue hair, and worrying about whether to point that out to him.

Web Comics

 * The webcomic But I'm a Cat Person is all in greyscale except for green spot-coloring...and the eyes and hair (or, in Miranda's case, hair decs) of certain characters, mostly Beings and their Masters. If you see someone with colorful hair, odds are, they're going to be important.