Many people of
European descent are born with hair that darkens over time. In early age their hair is
blond, but with the right
genes it may turn to dark brown, as in the picture on
my homepage.
Where this is common, a light colour is naturally perceived as a cute, childlike feature, just like big eyes, looking up from below, the appearance of innocence and playfulness, and pretty much everything else that is stereotypically considered sexy about women. Britney Spears professionally acts like a 4-year old and everybody loves it, teenage girls in particular.
Why? A longing for the innocence and safety of childhood? (Or to reverse it: personally, I'm attracted to dark and independent - does this mean I'm still a child?) Well, I don't know. But seeing Britney, Marilyn Monroe, or Madonna, makes me wonder.