Might be normal... my yellow tang changes color depending on the lights. (light yellow during the day, pale under acintics, and a red spot/whitish when totally blacked out)
My 3 stripe damsels did the same; being somewhat faded till I brought them to a brightly lit aquarium. And even then, the dominate damsel tended to have brighter/more defined colors.
I've removed him for being a snot.
The other damsel is now considerably better looking in terms of color.
Think it might be a mating thing?