thin bearded dragon

Question
We have two bearded dragons. I feed them a diet of crickets, pellets and
veggies. Lately, one dragon has become quite thin, hangs out on the the cool
side of the habitat most of the time, and has a darkened brown/black tone to
his skin color. I wasn't concerned at first because he remains quite active. I've
tried separating the two dragons during feeding periods to make sure the
other dragon isn't stealing his food, but the thin lizard ignores the food.
What's going on with my dragon?

Answer
He is most likely sick, a bearded dragon that wont eat, or bask in the right spot is usually sick. The only other option is that the one dragon has a large amount of stress on its body due to enclosure changes or being moved too much. Your best move is to take this dragon to the vet, it may have an internal parasite. I see that you have said that you feed them pellets, this doesn't have anything to do with how it is acting but you shouldn't have to feed them pellets if they are older.