Before you stop reading because you think I'm being terribly PC, please give me the benefit of the doubt. First of all, I'm well aware that there are differences between men and women, which probably go beyond those required for producing a baby in the first place. Second, I'm not saying that men and women should all dress the same. Like most men, I don't wear dresses (ok, there was one time...) and I wear my hair short. Like most people I conform to stereotypes too. And although I wear pink shirts or ties, even then I am conforming to a different stereotype, one which is explained without any hint of irony here.
It's not the colours themselves that are the problem, it's how people react to them. Researchers interested in this question took two groups of mothers and asked them to play with a toddler who wasn't known to them. The mothers with the beautiful princess dressed in pink behaved very differently than the second group with the little tike dressed in blue. The "blue baby" group encouraged the toddler to explore, to experiment with toys and be adventurous. The "pink baby" group were far more protective of the child. As you've probably guessed both groups of mothers were playing with the same baby - only the clothes had changed. It's frightening how something as simple as a colour can cause parents to start reinforcing stereotypes: teaching boys to take risks and girls to be timid.
The colours aren't the only problem. The pictures are worse offenders. Boys clothes have tractors on them. Girls clothes have fairies on them. And it doesn't stop with clothes. Almost every piece of baby equipment, from changing mats to lamp shades, can be gendered. Kirsty, as a scientist, was particularly offended by the dinosaur wall stickers that were only listed in the boy section of one baby catalogue. We wouldn't tell children what they should and should not be interested in based on their hair colour, so why do it based on their gender? If our baby wants to be the next Ellie Satler, why shouldn't it?
Another group of researchers had the bright idea of giving a troop of young chimpanzees toys to see what happened. The toys they used were dolls and cars. The female chimps chose the dolls, leaving the male chimps with the cars. So if chimps display gendered behaviour from a very early age, perhaps it is a genetic trait. There is a good evolutionary reason why female chimps should prefer the dolls: female chimps that want to play with infants (imitation or otherwise) are more likely to bring up their own infants successfully. The same may apply to baby girls. But an evolutionary pressure to be good mothers doesn't justify choosing clothes that encourage all baby girls to grow up wanting to be ballerinas.
There is of course a much more practical reason to despise gender specific baby clothes. If we have more than one child, we want to be able to hand down the clothes from the first one to the second. Perhaps I should start a baby shop specialising in white, beige, yellow and green.