Yep...agreed 100%. The darker color is just his skin changing color from the amount of light shined upon him. See how his head is lighter? It is because light is shining on it. His back half is shadowed and he is therefore matching the shades of light hitting various parts of his body.

It is pretty cool actually. If he were to be laying under a lattice with light shining from above, his skin would have a lattice discoloration. Their skin changes as camouflage on its own. Mine have had various stripes and shadows due to where they are positioned under lights and shadows. Their colors will re-even out once the shadow or light moves or the frog moves . He looks fine and quite cozy .