What determines how much and of what color tiles opponents that don’t move get each turn?
well, they get at least 1 gem of the color of their abilities
I assume that they get 2 when the ability is expensive? or when they are a higher “class” of enemy?
Nope, if you play the very first mission in HAMMER with 2 snipers you’ll see they earn 2 green each turn a piece.
It’s a set amount based on their abilities; I just haven’t watched very carefully to see if it’s always 2 or sometimes 1. But with the changed Soldier abilities scenarios with Soldiers and Venom are more annoying.