Mine do that too, at night. The first morning after I got them I came out to the living room, saw them like that and almost had a heart attack. But they colored right back up after they all woke up. I don't really know why they do it, but it doesn't seem to be harmful to them, and no one's outwardly sick in any way that I can tell, so I figured it's just something they do.
Now I'm curious; off to do some research on this.
Edit: found this. I learned something today.
Most smaller fish lose color at night. It's a defensive thing that makes them harder for night time hunters to see them while they rest. It's more pronounced in some fish than others and some recover their color faster than others.