Pink and blue were not gender signifiers until shortly after World War I. In the centuries prior to that, all babies were dressed in white gowns, which allowed easy access for diaper changes and could be bleached after wearing.
In 1918 the trade publication Earnshaw’s Infants’ Department wrote, “The generally accepted rule is pink for the boys, and blue for the girls. The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.
It wasn’t until the 1940s that manufacturers went in the opposite direction and decided that pink was for girls, and blue was for boys.