Answer:
Usually, the sand (yellowish, light brown) is clearer than the normal land (dark brown). Now, is normal that the mice that have a better survival probability are those ones with a colour similar to the environment in which they live. Those ones live for longer, and because of the natural selection, eventually, most of the population will have that color.
This is why the ones that live near the sand most likely are clearer than those that live near land.
