It would depend on the character. I know I have a tendency to like white hair on boys and...if you really had to make me choose, black hair on girls (although I care less about girls' hair colours - just give me whatever works with the character design). Preferably long hair either way.
Anyway, this one is about hair style / colors. What's your favorite anime / manga hair color, if you have one? and if so, give examples of characters with the color. ^^