the process that would make them go as dark or light would be because they and blend in better to no the eaten it is called "latrell Selection" so in the Beach mice would stay light because they look like sand but if they were dark they would be easy to see that is why they are not there same thing for the forest but the other way around
if the forest changed to more sandy it would let lighter mice live there to but the dark mice would still live because it is not all sand i like it would be equal amount of dark and light mice because they would both have there spot to hide and just as vulnerable if the switched spots
