I think his colour is just lightening up a bit because he's probably in better conditions then at the store. He was probably dark at the store because he was cold. Those 3 lines of lighter colour on his back is a pretty common markings for horned frogs. Good luck with your little eating machine.

P.S. all my horned frogs (actually, all my frogs) have UV lights, don't worry about it unless he's like 2 inches from it. As long as you give him a good dept in the substrate, if he doesn't like the light he will dig down.