My daughter asked this today are a part of a larger discussion about gender stereotypes. Anyone have insight?
The postwar period is when pink-girls, blue-boys had become firmly solidified in Western consciousness, but the transition occurred through the early 20th century. Throughout the 19th century, infants of both sexes were dressed alike, which often meant in white. Pastel trimming was then used to add a bit of color - you can find magazines supporting blue for boys and pink for girls, pink for boys and blue for girls, and sometimes combinations including green and yellow, all of these representing individual editors' tastes rather describing a societal rule.
The solidification goes hand-in-hand with the phasing out of dresses for very little boys, which also occurred gradually during the early 20th century. In part this has to do with changing notions of masculinity, but it's also related to advertisers wanting to increase parents' need to spend money - you can't hand that down to your son, it's for a girl!
Jo Paoletti's Pink and Blue: Telling the Boys from the Girls in America is the best thing you can read on the subject.
I would also add that pre-WWII, in Europe, blue was often associated with girls because of the connotations with the Virgin Mary (depicted wearing a blue cloak). Conversely, since pink was a red tone, and hence a "stronger" color, pink was frequently associated with boys' clothing.
Edit for clarification. Source
hi.. you may be interested in this earlier post