Well as bam says up there it’s about blue being for a baby boy and pink for a baby girl, however…
It actually used to be pink for a boy and blue for a girl. Red was seen as a masculine colour (I think it was seen as kindof powerful and passionate, the colour of blood and war and shit like that), so a lighter red (pink) was appropriate for boys. Blue was associated strongly with the virgin Mary so you had light blue for a baby girl.
I’m not sure why it changed really!
Also… i think it kind of went away as a ‘thing’ for ages, or maybe wasnt that much if a thing at all, not sure, and then when we worked out how to tell the sex of a child during pregnacy manufacturers of boys and girls clothes really went to town in the gendering of colours so that expectant mothers of a second child (of a different gender than the first child) would feel they couldn’t just use hand me downs from the first child.
I think this is mostly true!