It is usually stress.
Did u get it at a pet store?
If u did, they r usually stressed from being switched from their original habitat to the pet store's habitat then suddenly to ur habitat. If it is stress, they will color back up again within a few days-a few weeks.
If they don't color up after a few weeks, it's prob cuz their parents weren't colorful either.
It wasn’t until 1853 in London when aeration and filtration of water was understood, that people
were able to keep fish as indoor pets.