Where I live we have a river that produces two distinct coloration's of the same species of may fly.

The river runs through a red sandstone canyon. There are sections where the river bottom is made up of deep red and pink gravels. A PMD variant has developed with a distinct pink colored body. The river also has PMD's with the usual olive color.

Perhaps the color of a may fly is dependant on natural selection, I could see how a pink colored nymph could hide well in the red gravel sections of the stream, where an olive one would stick out.