My black skirt tetra's tails changed colour?

When I got them their tails weren't that black, then a few days later one of them was black and I figured it was just to show who is dominant, then a few days after that the other five had black tails. Now today all their tales are pretty white. I thought that they lost their colour over age, not any other reason? I know that a lot of colours change in fish depending on mood/age/care level, but I'm not sure about these fish. What's causing this in these fish?