That was to ensure they didn’t steal the underwear.
This is at some level is likely meant to encourage some kids to be unsure about what underwear to wear. Or work towards that.
What these Left wing twits don’t seem to know history:
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.