They can change color as they mature, however they can also fade if they're not well. If the water parameters in your pond are good and he's swimming and eating normally, he may just be changing color. You may want to put some plants in or around your pond to provide it with some shade so the algae doesn't get as bad. Where are you located? My pond is under ice. The only reason I can see my pond fish is because they're in the basement.
