Pink was originally thought of as a strong colour and boys were dressed in pink. Blue was associated with the virgin Mary, and was used as a feminine colour for girls.
There is an excerpt from
Time magazine from 1927 when Princess Astrid gave birth to her child, it was reported that the nursery had been hopefully decorated in pink, for a boy.
You may have heard of Gainsborough's 'Blue Boy', he also painted a companion piece called
'Pink Boy'
It is only during the 20th century that pink has changed to being associated with girls.
Ladies' Home Journal article in June 1918 said, "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." Other sources said blue was flattering for blonds, pink for brunettes; or blue was for blue-eyed babies, pink for brown-eyed babies, according to
Paoletti. (Pink and Blue: Telling the Boys from the Girls in America - Jo Paoletti)
From Smithsonian.com
In 1927,
Time magazine printed a chart showing sex-appropriate colors for girls and boys according to leading U.S. stores. In Boston, Filene's told parents to dress boys in pink. So did Best & Co. in New York City, Halle's in Cleveland and Marshall Field in Chicago.
Hope this helps.