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What are the clothing colors for baby boys and girls?
According to a traditional color scheme, which is of unknown origin, baby boys are properly dressed in pink clothing and baby girls in blue, although in some parts of the country, particularly in the Southern States, this symbolical color arrangement is reversed and baby boys are dressed in blue and girls in pink. One writer says that blue was assigned to girls because that was the color assigned to the Virgin Mary and the royal house of David to which she belonged. At any rate, blue and pink have become associated with babies. When friends are notified that the stork has paid a visit to a home the announcement cards are often decorated with blue ribbons if the baby is a girl, and with pink ribbons if it is a boy. Apparently, however, this traditional color arrangement based on the sex of the child is giving way to more practical considerations. In the issue of Forecast for May, 1927, Mollie Amos Polk says on this subject : "According to the buyer of one of the most famous shops pink and blue are now used interchangeably for boys and girls. Pink, however, since it is universally becoming and will stand frequent tubbings, is much more popular for both."
According to the 8 October 1914 Shoe and Leather Reporter article Children's Shoe Fashions for Spring and Summer 1915:
shoes trimmed with blue for baby boys, and pink for baby girls are shown in the spring samples
Like fredsbend commented, there are lists of additional sources on Wikipedia. Sources saying "blue for girls, pink for boys" can be found up to about 1941; however, it was not a strong tradition. During the same time period, as the above shoe reference shows, people also dressed boys in blue and girls in pink.
Another clear counter-example is the article Women's Part in the New Renaissance Century Magazine, April 1923:
Mothers take good care to discover the suitable color and adornment for their little ones, blue for the little boy, pink for the little girl...
The strongest evidence I can find for a tradition of pink for boys, blue for girls, is that there was criticism of president McKinley's wife for sending blue booties to former president Cleveland's wife on the occasion of a boy being born:
as all the world which has had experience in such things knows, blue boots are for girls and pink for boys
Omaha Daily Bee 7 November 1897 (similar articles 6 November 1897 in Desert Evening News, the Salt Lake Herald and Blackfoot News)
Additional articles recommending pink for boys, blue for girls include:
The earliest article, Styles Here and Abroad, The Sun, New York, 09 January 1888:
Pinks and reds are the colors for boy babies, blues and creams for girls
A Word about Babies: Appropriate gifts St. Paul Daily Globe 22 October 1889 (the advice spread to several other newspapers a month later, the Hickman Courier, the Ohio Democrat and the Macon Beacon):
pink for boys, blue for girls, the gossip says
About Fall Fashions Evening Star, Washington, 20 September 1890 (same article in Indianapolis Journal):
white ribbon for the first three months, afterward pink for a boy, blue for a girl-clover pink for a blonde boy and very pale blue for a dark baby girl
Article in The Sun, New York 03 July 1892:
pink for a boy, blue for a girl, according to French fashion
Baby's Wardrobe Lawrence Democrat 29 September 1893:
"Pink for a boy and blue for a girl" is a generally accepted dictum
For Small Fry Salt Lake Herald, but with New York dateline, 19 April 1896 (also published in The Morning Times, Washington, and days later with the title For Small Children in the Norfolk Virginian):
Pink pique is also used for small gentleman in the baby stages...
Blue being a girl's color the sky blue pique is not used for boys
Article Omaha Daily Bee 26 July 1896 (same article in Salt Lake Herald ):
generally pink for a boy and blue for a girl
Article in The Daily Morning Journal and Courier, New Haven, 01 March 1900:
it's pink for a boy and blue for a girl
An Arbitrary Rule The Denison review, 29 July 1902 (also published in The Columbus Commercial, the Willmar Tribune, the Barbour County Index, and The Columbian):
you know blue is only for girl babies, pink's for boys.-Philadelphia Press
Sending Christmas Gifts Sisseton Weekly Standard, 21 December 1906:
If you are sending a gift to a baby, tie it up with blue ribbons if the baby is a girl, pink ribbons if it is a boy
Blue for Girls El Paso Herald, 11 December 1912:
Dear Miss Fairfax: To settle an argument ... what colors are used for babies..."
...custom has given blue to the girl baby and pink to the boy.
Birth Announcements The Monett Times 14 February 1913:
For boys a pink border, for girls a light blue
Baby Books Bridgeport Evening Farmer 17 March 1913:
blue for girls and pink for boys
After a reader disagreed with columnist Cynthia Grey's (pseudonym) advice "pink for a boy, blue for a girl", Grey responded in The Tacoma Times 28 November 1916:
According to the authorities at the public library...Pink is for boys and blue for girls. This is an old Dutch custom. When a boy was born a pink ball was hung out and when a girl was born a blue ball was displayed.
So in conclusion, though numerous sources can be found starting around 1888 saying that "pink for a boy and blue for a girl" is traditional, others expressed that the tradition was visa versa.
A quantitative study was done in 1921. See Correct Color for Birth Card Announcements in volume 89 of The American Stationary and Office Outfitter, 6 August 1921, page 18:
Our questionnaire replies showed cities totaling 12,000,000 people using blue for a boy, cities totaling 6,000,000 using pink for a boy.
The study was conducted by the National Association of Steel and Copper Engravers with the stated purpose of standardizing the colors and was published in several other journals, such as Geyer's Stationer; Dry Goods Economist; Walden's Stationer and Printer and Modern Stationer and Book-Seller