Typically, when a bearded dragon begins to turn yellow it is a sign that the reptile is not feeling well. If a bearded dragon experiences any abnormal changes in its skin, it should be seen immediately by a veterinarian.
Yellow skin disease is caused by an aggressive fungus that invades the upper and internal layers of the skin. It can cause discoloration, lesions and even ulcers underneath the skins layers. In severe cases, the fungus can reach the internal organs and cause swelling and inflammation. In some cases a dragon's hind legs will turn a pale yellow as a sign of an illness that should be checked out.
This illness spreads easily among other reptiles, and treatment should start immediately any symptoms occur.