Fish can regulate their colors on a regular basis.........I have a Flame Hawk, and obviously his name denotes his very bright red/orange color, and at night after the light has been off for some time he can become pale and a little white. I think it mainly when the fish slows down and gets ready to sleep.
Another reason is the lack of variety of diet.....any fish can loose color. I would suggest some type of small pellet food with spirlina (spelling) in it. This usually helps with the fish's color. I recommend Vibra grow....my flame hawk and anthias eat it like its going out of style.
GOOD LUCK!