There isn't a way to directly change an actor's sprite. One thing you could possibly do though is attaching MOs to the actor, but you'd want to watch how many you stick on since it'll get laggy if you have too many, and they'd only really be able to stick to the outside of the actor's body without finicky (and probably slow) tricks. The closest you'd get to a color change is using the
FlashWhite() function.