I think it's nothing to worry about. When angelfish are excited, in breeding condition, hunting, that kind of thing, their colors generally intensify. If they are scared, intimidated, stress due to lack of hidey places, etc., they may blanch and lose some color.
If they stay faded for weeks on end, but appear otherwise healthy (swimming fine, eating OK, etc.) think about adding some plants so that they can retreat as necessary.
But generally, fading is not a sign of illness. It may be an indicator of some other problem, which if left to go on too long, might cause health damage from the stress. So I guess what I'm saying is fading is a secondary reaction, and not, in and of itself, a problem. It's natural.