Good question. I've heard that a guy's preferences in the female reflect those of a female in his early childhood that he liked. I'm not saying that it's true or not. At an early age I preferred brunettes and then in high school it was blonds, but now I seem to prefer brunettes again. I don't think hair color has anything to do with personality, but it seems that I am now more attracted to brunettes than woman of any other hair color.
I like blonds primarily (though sometimes brunette does suit the individual girl better).
After that I like redheads.
In this category I do like some more exotic colours, like when girls dye their hair pink and such. Normally they have a better idea of what suits them, but I do think bright exotic colours look good.
Brown - I find brunettes (hair colour, not personality) to be boring. A rich light brown can look good though.
Black - Put some colour in there.
Ok one thing I do have to say is that all of the above for the most part applies to caucasian girls. Girls of other ethnicities I find tend to pull off the brunette look better though I think it's because blond and other looks too unnatural on them. I'm sorry if that's not very helpful.