Try this experiment on your fish some time.
Have two 5 gallon buckets half or more filled with fresh pond water, one bucket black and one white. Put the fish in the black bucket for a few minutes and observe the color. Then put the fish in the white bucket the same length of time and do the same. The fish can change their chromatophores fairly rapidly and the same fish can look markedly different in coloration.
I have found that fish I want to transfer between my (close) ponds remain much more docile if I keep them in a black bucket as opposed to a light colored bucket that lets more light in. They are a lot more jumpy and agitated in a light colored bucket.
Slight modification to above. Place to pieces of plastic, each roughly two feet square, on bottom of pond or tank holding fish. Have one piece white and other black. Allow fish to swim back and forth over them at their own pace. Note coloration changes. Smaller Bluegill can make changes faster. Warmouth are better yet.
