It’s not just “a” girl color, but the international spokescolor (yes, a made up word) for the female gender.
Here's a great post from Kyle Wiley of The Good Men Project (re-blogged via the Huffington Post, but hey, Arianna's rich enough). My favorite line:
Made up words are the best, because like all custom-made items, they fit better than the off-the-rack-versions. That is exactly the idea I have been trying to get across, less articulately, when I talk or write about pink and blue. Blue is NOT a spokescolor; pink is a spokescolor. Why is that, do you think? Is there something magical about pink itself? Mais non.
The magic is one of the oldest known superpowers: giving birth. Stay with me, friends. Here's how I see it: Women used to be powerful because they gave birth. The only way men could be more powerful than women was to control reproduction -- through marriage, through rape, through laws about birth control and abortion. But none of that transfered the magical power from women to men, so a cultural solution emerged instead. Make birth dirty, make sex a sin, make women dirty, weak sinners, lower than men because of their magic power.
Now all you have to do to maintain male superiority is make sure they are not tainted by anything remotely effete or feminine. Punish homosexuality. Raise little boys to be not-girls. Ridicule boys --and men-- who cry, or who are unathletic, or who like pink. It's a small price to pay for a place at the top of the social order.
Why have women put up with this? Many reasons, including a need to protect their offspring, their own survival and this complicated force called "hegemony", which results in acceptance of the dominant culture even when it works against you. (Kind of a cultural Stockholm syndrome.) But all is not lost; there are men and women, mothers and fathers, who believe that all humans have magical powers of love, imagination and creativity, and that humanity will benefit when every baby is valued for its potential to love, imagine and create, not its role in human reproduction.
Peace. (Steps off soapbox, returns to her index cards.)