You've probably seen the bright yellow and black Foxface. This is a more expensive and somewhat more rare Magnificent Foxface (Siganus magnificus). This particular fish is a ravenous herbivore, and will mow down nuisance algae in a reef. When stressed or frightened, their color changes. Special care should be made to avoid the spines on its back, as these carry a serious sting. Reef-safe.
Foxface / Rabbitfish can change their markings based on mood. I've seen them turn nearly entirely black when spooked, but given time they will come back out into the open and their normal coloration returns.