It doesn't really matter, but I prefer brunette or black hair, especially if they have tanned/olive/darker skin. It's such a sexy look for me. Next would be blonde followed by redheads. But I must say, there is nothing sexier to me than the sight of a redheaded woman's natural pubic hair/bush.
This is especially true in light of the vast majority of women going completely bald nowadays. I like some hair down below, and a redhead with some hair down below is aces in my book!