Tiger Barb fish fading stripes, then reappearing?

We bought 3 tiger barbs, and 3 sword tails. They all do fine in a 20gal aquarium. We've noticed that 1 of the tiger barb fish fades it's color. it lightens and darkens it's stripes, but I don't know why? Is it normal? I find it kinda cool Lol. I hope it's healthy.