Originally Posted by al_a
One thing that has not yet been mentioned is that most freshwater fish have the ability to change color in a short period of time to match environmental conditions.
Thats very true and a very good point. When I still had the trout in my aquarium they would change colors pretty often, especially when feeding. The more dominate fish would get very bright and the less dominate would turn almost black and this happens in the a matter of seconds. When they were done feeding, they went back to the normal colors of green and purple.
What the fish are eating will also make the colors change. When I was feeding just trout chow the fish were silvery purple and looked like most stocked fish do. Once switching over to a better food and feeding different kinds of food, the colors really perked up. The more natural and varried a diet they get the better the colors will be. Thats what I would say is happening in your picture. The shinier trout is a newly added stocker where the other fish has been eating bugs and whatever else and has effected the color. They will also change colors slightly pending on what color the stream bed or surrounding area is. Most fish will do this too, go home and play with an aquarium and be amazed at how you can change your fishes colors.