Do trolls skin turn black when they become adults, or is that just how Hussie drew them? (Also, do you mind listing all possible blood colors for trolls? You don't have to if you don't want to, but it'd be like a refresher for those who don't remember.)

The black forms for the adult trolls is just a stylistic approach, just like the human guardians don’t have all of their face’s depicted. Also, all the troll blood colors have been revealed in-canon.