Though they did talk about how it used to be blue for girls and pink for boys and then it flipflopped. Perhaps because it was not so pervasive. One panelist talked about the difficulty of finding something neither pink nor blue and other panelists were surprised -- no yellow or green?
And Scarlett O'Hara standing out in her mourning at the ball because of the color.
Sumpturary laws, forbidding people to dress above their station. Which was a revenue source because you don't pass laws against things folks don't do. Also a way of showing off your wealth by your ability to pay for it.
Also symbolism in objects. One panelist talked about the book Three Hearts and Three Lions -- straight out heraldry, and on its cover it had the title, a heart and a heraldic lion. Gives it right away: about courage and love.
I asked about obscure and self-invented imagery, and the panelists agreed that you had to like to exposit to pull it off, and be good at it -- some writers would be really well-advised to stay away.