It really isn't anything to worry about, unless they stay pale for an extended time. This would indicate some sort of stress. Many fish change color to some extent, those stripes are a sort of natural camouflage, in the wild angels live among tall thin plants, this pattern helps them to blend in. I've noticed silvers are nearly always faded when I turn on the lights in the morning, and color up after a few minutes.
Fading and then going back dark in a tank with a krib probably means the krib is causing a little stress. I wouldn't worry too much unless you start to see some torn fins.