I've had a mag foxface for about a year now. I think his coloration is closely correlated with his comfort level in the tank.
I bought him off of LA DD originally so his colors were very bright at the beginning. Even though he ate readily and had a good diet (Rods, Nutramar ova, PE mysis, nori), his colors faded over the first month or two. Interestingly, he never quite understood that I was the food source instead of something to be scared of. Within the past month he has finally started coming up to the glass when I show up with some food, so I think he is finally getting it... and he has colored up substantially with no change in diet.
FWIW, I sort of noticed the same "coloring up" once my dispar anthias finally got used to the routine.