Ditto - double check your temperatures and get a good UV light like the MegaRay. I've seen photos of the color difference Gnuby went through, and my Gizmo also went through a huge color change when I switched him over to a MegaRay. I've noticed that my Uros really color up the hotter they are, i.e. when they are basking directly under their MegaRays, they are the brightest.

Here is my Gizmo before having a MegaRay:

And after using a MegaRay: