Yes, a couple of my fire bellies have changed color--one that likes to hide in a clay pot-- I buried on it on it's side, with about half below the substrate surface. My substrate is very dark & that frog has become very dark--I have a hard time finding him sometimes! And another of my frogs loves to sit on top of a fake 'cave' from the pet store. It has lime green "leaves" on the top and she has become lighter green & a more lime shade. She sits there so much I renamed her to Limestone.