Blue is for Boys and Pink is for Girls

In early 1800's, boys and girls were dressed alike in white from birth through early childhood. Babies wore "long clothes" which were white cotton dresses that extended beyond the length of the infants' body.

In the early twentieth century, ideas about child-rearing changed, and developmental stages such as crawling were encouraged. Children's clothing was adapted to allow for more movement and more colors were introduced.

Originally, pink was designated for boys, as it was thought to be the stronger color. In Christian tradition, red was associated as male, and its ‘little’ sibling pink was used for boys. Blue was associated the Virgin Mary and therefore considered feminine.

After WWI, blue was used extensively for men's uniforms. Therefore, blue became associated as more of a masculine color. From the 1940's onward, pink was pushed as a woman's color. "Think Pink" was the marketing slogan to convince women to embrace their femininity. Dressing girls in pink reinforced this role by conditioning with a set of rules that started with little girls, made of "sugar and spice and everything nice."
Today, those gender specifications hold strong, with pink designated for girls, and blue for boys.