The trope You Gotta Have Blue Hair is "unnatural hair colours".
Now, while your logic is part of the reason why we have a big concerted effort in Long Term Projects to clean out these appearance tropes, it doesn't really work here: Hair colours that can happen in fiction but not Real Life are tropes - things that can't happen outside of fiction are always tropes, really.
edited 15th Nov '13 10:27:58 AM by SeptimusHeap
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman
Yes, but people can have these hair colors, even if they are unnatural. Its just that, if no one in-universe comments on it, and it serves no function in the plot or story, it isn't really a trope.
Look at Hayley Williams, for example.
Unnatural hair colors are noticeable.
Unnatural hair colors are used to make the main characters more noticeable.
I believe this is indeed a trope.
You'll note that this appears most often in anime—because in real-life Japan, pretty much everyone has black hair. It's like Mikuru said, it's about making the character more noticeable and memorable.
Yeah, the presence of a general thread for cleaning up appearance tropes makes this thread kind of a redundant waste of space.
"Yup. That tasted purple."
This is one of those things that, like eye colors, isn't really a trope, most of the time. And for when it is, there are plenty of actual subtropes.
White Hair, Dark Heart-that's a trope.
But just slapping instances of characters with unusual anime hair colors onto the examples page, or onto the character page, is a waste of space.