A June 1918 article from the trade publication Earnshaw's Infants' Department 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 colour, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.”
A blog by Grammarly notes that:
"Gender identification by colours began in the 19th century in the Western world. Before this, pink and blue did not hold any gender specific connotations and there are numerous examples of men wearing pink outfits and girls wearing blue; one French author, Xavier de Maistre in his work, A Journey Around My Room published in 1794, even recommended that men choose to paint their rooms pink and white to improve the mood."
The reasons are obscure, but Lauren Sadler suggests cultural influences of the day:
“Throughout the 19th century, children of both sexes were dressed in long white gowns. When gendered palettes came into vogue in the first two decades of the 20th century, boys were assigned pink and girls blue. This was a nod to symbolism that associated red with manliness; pink was considered its kid-friendly shade. Blue was the colour of the Virgin Mary's veil and connoted femininity”
Fashion scholar Valerie Steele notes that: "In the 18th century, it was perfectly masculine for a man to wear a pink silk suit with floral embroidery. Pink was considered slightly masculine as a diminutive of red, which was thought to be a warlike colour"
And this is still the case, when towards the end of the 1925 novel The Great Gatsby, Jay shows up to lunch with his mistress and her husband in a pink suit.
In a recent 2006 movie, costume designer Catherine Martin says she based the suit worn by Jay on the pink seersucker of the day, traditionally worn by the hired help, but which some of the wealthy New Yorkers decided was comfortable for summer.
In 1927, Time magazine printed a chart highlighting gender-appropriate colours for girls and boys according to leading U.S. retailers. Filene’s (in Boston), Best & Co. (in New York City), Halle’s (in Cleveland), and Marshall Field (in Chicago) all advised parents to dress boys in pink and girls in blue.
This article was inspired by news of the birth of a daughter to Princess Astrid of Belgium, who had decorated the cradle in pink, the traditional colour for boys in that country. And blue was still a “girl” colour in Switzerland.
Jo Paoletti in her book Pink and Blue: Telling the Boys from the Girls in America, says that pink denoted health, as in the phrase “in the pink”: She says that “Young men and women might wear pink clothing; old men and women did not.”
"Pink and blue were suggested as interchangeable, gender-neutral nursery colours,' appearing together in many of the clothes and furnishings found in the baby's room"
The change seems to have happened sometime in the 1930s, driven by American manufacturers and retailers, and pushed hugely after the second world war, so that by the 1950s, pink for girls, blue for boys was firmly established as “traditional” although of course it was nothing of the kind!
As Paoletti notes, in 1927, colour preferences in shops in major US cities had 6 favouring pink for boys, blue for girls, and only 4 favouring the reverse.
The Taylor survey of its New York area customers in 1937 revealed that about three-quarters of them believed that pink was for girls and blue for boys, and the rest preferred the reverse.”
Paoletti comments that: “By the 1950s, pink was strongly associated with femininity. However, that connection was neither universal nor rigid; boys could still wear pink dress shirts and have pink frosting on their birthday cakes without risking gender confusion or public censure, and girls wore many colours besides pink”
But matters were becoming more rigid. In1959, the New York Times quoted a children's clothing buyer: "A mother will allow her girl to wear blue, but daddy will never permit his son to wear pink."
There were certainly commercial pressures. Paoletti notes that: “The more baby clothing could be designed for an individual child—and sex was the easiest and most obvious way to distinguish babies—the harder it would be for parents to hand down clothing from one child to the next, and the more clothing they would have to buy as their families grew.”
But the change was still gradual and there was never a point when there was a sudden break with the past, but just gradually, over decades, the colour reversal took place.
This become the norm, and so entrenched, that the cultural memory of the reverse became forgotten, and it was firmly believed that this was the “traditional” colour coding for boys and girls.
But as Jo Paoletti notes, "the conventions of 2010 are nearly the reverse of those in 1890"