There was a study about how children gain colour preferences. The researchers asked very young children to pick balls with different colours. The three-year old group picked colours quite randomly, boys often picking pink and girls blue and so on. By age four or so this changed and the boys started avoiding pink and purple balls and the girls choosing them.

The explanation is that at a certain age kids want to know how their group (boys, girls) behaves, so they learn the cues for that from the world around them. But because they are still very concrete in their thinking, the meaning of things like colour preference becomes very policed. I knew a little boy who at age four or so firmly believed that when he put a necklace on he became a girl. The princess dress etc. may be something similar.

Once kids understand that their being boys and girls is stable and not depending on all that stuff the pink phase tends to be over for most girls. Of course now schools teach that being a boy or a girl might not be stable so perhaps in the future teens walk around in pink princess dresses?