Last Saturday I bought a new fbt. This fbt had a very bright green color, just like the one I already had. But the one I already had changed color to more brown. The new green one is also getting a bit darker. What's happening? Am I doing something wrong? They live in a Exo Terra vivarium with real plants, lot's of water and places to hide and rest. The tank has a compact top with 2 low energy lightbulbs (2x 11 Watt) I want them to be green again