My answer to a previous question may help you:

It used to be acceptable to dress young boys in dresses. Nowadays dresses are considered "feminine," and dressing a young boy in a dress would be cause for some side-eye. Why did this change?

While from the title it's clearly not just about the color-coding, today's color-coding is a result of the ideological changes concerning how much gender-differentiation was considered appropriate for children in the past.