Not that I have any massive preference or hair colour will ever be a deal breaker (it most certainly isn't) but my favourite hair colour on women tends to be brown.
I voted black but really, a dark brown is good, too. I like it dark is what I'm really saying. With redheads as a very close second. That's not to say that there aren't blondes I'm into, because there are. However, I lean more to darker women (hair and complexion) than the classic light-skinned blonde. In the end, though, hair colour isn't going to be a deal-breaker though some dye-jobs might be.
"Natural" ... top and bottom (if any on bottom...lol!)