My Tiger Barbs fade on occasion. In my experience, I have noticed that if the lights stay on too long (or they are too bright) they will tend to fade a bit. (just a bit stressed, I guess) They are usually OK by morning.
What about their food? Could it be some sort of deficiency?
I feed mine regular flakes, but once or twice a week I give them some frozen blood worms and once or twice a week some frozen brine shrimp.
For dessert, some nights, I give them "ColorBits".
You can check this site for other ideas.
I hope this helps.