I have a red veiltail, and he loses color under his chin when he needs his water changed, or just isn't feeling well in general. I change his water and drop a few drops (~5) of Bettafix in his tank when that happens and he's good to go. The only real issue I've had of late is with my conditioner basically going bad (from what I can tell), so he's been a bit off-color and he bit off a portion of his tail because of it (I think).
He gains his color back, but he goes a silver-gray there, and he gets spots nearby. Once he's happy, his color comes back within hours. It's a great water/health indicator, I find.