MillyR - I'm not strongly invested in the idea that liking pink is more predominant in girls, but I don't think it is necessarily thrown out of the water by your argument that it is only recent.
The 'girls like pink' thing may be like cheesecake, or mumsnet, a recent invention that becomes popular because it manages to press our buttons.Cheesecake combines fat and sugar, crunchyness, juiciness etc..in a form not found in nature, but many people find it irresistable because it combines loads of hard-wired tastes. Mumsnet is a recent invention, but its addictiveness comes from its ability to meet human needs for contact, recognition, love of gossip etc.. which are all innate etc...
Personally I think liking/noticing pink (rather than say grey or green) is likely to be innate for most people, and may be marginally more predominant in girls rather than boys, who knows.
The companies selling pink stuff for girls have maybe stumbled on a way to make money rather than invented it out of nowhere. My experience as a parent is that my boys liked pink, until they realised at school they 'weren't meant to'.
I think a lot of perceived differences between girls and boys, males and females may be like this marginal differences in frequency of a trait one way or another, amplified by social forces.