If he isn't keeping his red color it is because he is unhappy in some way. A grouper will almost always eat, mad or not. It could be a number of things. Unhappy with his tankmates, possibly intimidated, still getting settled into the new situation (some groupers can take awhile to feel comfy), unhappy with water conditions (do you know what all your levels are? ammonia, nitrite, nitrate, pH, salt, etc?) There is something that needs to be fixed/changed for him to feel ok in your tank.