katharos

I think anime and manga characters in general do tend to have hair and eye colours that function as visual 'tells' - they codify a lot of basic information about the character, such as purple eyes signifying 'magic' beyond that world's norm, (Sanzo, Kamui), and dark green hair a reserved, often refined personality. This then allows the mangaka to move straight into demonstrating more detailed aspects of the characters personality.

This isn't a hard and fast rule of course - it's very easy to come up with contradictions. It's just interesting.