Although fish do experience color change its due to there chromatophoric cells unlike coral where pigment is derived by the zooxanthellae. Color change in fish is much more rapid and reversible (At least in certain species), unless he/ she is maturing or changing sex which I don't think is possible with damsels I'm stumped. Maybe some of the fish keeping experts can chime in, have you been feeding him anything funny
?
Levi