A recent essay in the New York Times by Claire Cain Miller, entitled How to Raise a Feminist Son, claims that:
Until the mid-20th century, pink was the boy color and blue was for girls.
It, in turn, cites an Ask Smithsonian blog post by Jeanne Maglaty.
How well-grounded in the historical evidence is this claim?
According to the 1930 version of Stack Exchange, Popular Questions Answered:
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
This claim originated in a 1987 paper by Jo Paoletti, a researcher in American Studies, and was presented in expanded form in a 2012 book by the same author. In these publications, Paoletti did not argue that pink and blue had been reversed, but only that their use had been inconsistent prior to the 1950s. It is unclear how and when this initial argument morphed into the popular, widely repeated idea that the gender associations of pink and blue were uniformly reversed in the early 20th century. While some text fragments found by Paoletti and others are compatible with a scenario of inconsistent gender norms, the idea of a complete reversal has no basis in historical evidence and seems to have spread as a sort of urban legend, eventually making its way into the scientific literature (see here for examples and discussion).
In a 2012 letter to Archives of Sexual Behavior, Marco Del Giudice performed a Google NGram search in English language books (US and UK) from 1880 to 1980 and found no evidence of inconsistent usage during this period; in contrast, there were many instances of the familiar female-pink and male-blue associations. Based on this finding, the author suggested that the text fragments that are usually cited in support of Paoletti's claim may have been unrepresentative of the cultural norms of the time, or (at least in some instances) have other, less obvious explanations (e.g., misprints, satire).
Jo Paoletti and biologist Anne Fausto-Sterling replied to the letter in a Huffington Post article, although they did not provide additional evidence. In the same article they challenged previous speculations that sex-related preferences for pink and blue may be partially rooted in biological processes.
Note: to date, the issue of a possible biological basis for pink/blue preferences remains unresolved. The relevant peer-reviewed literature includes:
A 2007 paper by Hurlbert and Ling that found a pattern of sex-differentiated color preferences in different regions of the visible spectrum (blue-green vs. reddish-purple);
A 2014 paper by Sorokowski et al. that found similar sex-differentiated preferences for colors (along a blue vs. red-purple axis) in Poland and in the Yali of Papua New Guinea;
A 2015 paper by Wong and Hines that found no robust associations between color preferences and other sex-typed preferences (e.g., toys) in toddlers.
UPDATE (June 23, 2017): Here is a link to a new letter in Archives of Sexual Behavior (Del Giudice, 2017). The letter updates the 2012 Ngram search and supplements it with information about newspapers/magazines from Chronicling America and Wikipedia. It also reviews recent studies of sex differences in color preferences and summarizes current evolutionary hypotheses in this area.
Not in Russia - boys/blue & girls/red association has been around for 200+ years.
The Soviet/Russian official theory is (wiki/source):
Обычай перевязывать новорождённых мальчиков голубой лентой, а новорождённых девочек красной восходит к вышеупомянутому указу Павла I награждать каждого родившегося великого князя при крещении орденом Святого Андрея Первозванного, а великих княжон — орденом Святой Екатерины
Translation:
The custom to tie newborn boys with a blue ribbon (and red ribbon for girls) stem from the law that every Grand Duke is decorated with the Order of St. Andrew the Apostle the First-Called at christening (and every Grand Duchess is decorated with the Order of Saint Catherine).
The St.Andrew ribbon is blue, St.Catherine ribbon is red.
The aforementioned law is the statute of the Order of St. Andrew signed by Paul I of Russia on 1797-04-16.
In an episode of QI (youtube) they address this, their website I will quote below:
Until the 20th century toddlers of either sex were normally dressed in white, but when colours were used, boys were dressed in pink. At the turn of the 20th century, Dressmaker Magazine wrote: 'The preferred colour to dress young boys in is pink. Blue is reserved for girls as it is considered paler, and the more dainty of the two colours, and pink is thought to be stronger (akin to red).' As late as 1927, Time magazine reported that Princess Astrid of Belgium had been caught out when she gave birth to a girl, because 'The cradle…had been optimistically outfitted in pink, the colour for boys.'
