“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.” —Ladies Home Journal, June 1918.
That’s from Smithsonian Magazine. Another interesting factoid: kids don’t realize gender is permanent until about age 6 or 7. So it’s perfectly normal that Ian likes to dress up like a princess from time to time. (Not that I ever thought his gender identity is unusual; he’s far more likely to dress as a firefighter.)