Traditionally, reds and pinks - the colour of blood - were signs of virility. Blue was seen as girlish, possibly because in Christian tradition (especially Catholic), blue is trademark of the Virgin Mary.
Likewise, it was always traditional for children to wear skirts, regardless of gender. It wasn't until a boy hit puberty (though 'boy' only really means servant - all children used to be called 'gerls') that he'd be 'breeched', e.g. given trousers to wear.