Marine fish change color with stress and mood and food and all sorts of wonderful reasons. I wouldn't worry too much about it until the damsel is crammed in a corner acting "just not right". Then you know the critter might be sick.
BTW, to answer your questions, damsels eat just about -everything-. One thing they might really like is some seaweed you can purchase dry at the LFS, or the frozen marine formulas that have seaweed/sponge base (Ocean Nutrition Angel formula and Formula II are good stuff). Damsels are omnivores with a tendacy towards herbivory in most species, although they will eat brine and other small crustatans as well.
~~Colesea
