are colonial cultures even a thing? I only know of american existing and I'm not even sure if it actually ever shows up (I've never seen it)
Nope. You would THINK they would be in the next expansion, but that "culture" feature doesn't amount to much.
It would be simple to implement, too... Colonial Nations with a special name get a special culture (Thirteen Colonies --> American, Vinland --> Vinlandic, etcetera), Colonial Nations that don't generate one (British Mexico --> British Mexican), and players can always edit it (Secret Denmark --> Secret Danish).
What I really want, though, is a much more complex system that involves slavery and the castes found in the Spanish world. I'm thinking of a system where you basically have to trade keeping a mestizo culture around (which is basically Accepted) or wiping them out and losing Development, and having the option to increase Development in cash crop provinces at the cost of gaining slave cultures. It would be the player's choice as to how they "color" their colonies, and would vaguely simulate the reasoning that, historically, went into racial settlement in the Americas.