I think he just may be adjusting, but keep an eye on him. They darken for two reasons usually, to match their backgrounds/homes and due to stress. If he continues to eat and poop (normal poop) and his weight doesn't appear to drop, then that's a good sign. It could be that the pet store didn't give him the appropriate amount of calcium and vitamins, or perhaps their water quality was not good. It's hard to say. I think he'll improve for you.

Keep us posted. I'll check in from time to time to see how it's going.