The reason behind the different colors of mice is due to genetic mutations which caused the different variations
Changes In Gene Pool
In this scene, there is little to no food for these birds, meaning only the fittest will survive
A different colored animal got lost exploring and ended up dying because it wasn't fit enough and wasn't able to reproduce. However, the dark animal was able to survive because it was fit and it was able to reproduce.
In this scene, a white hamster and a dark hamster mate, causing a new type of colored hamster to be born. This is an example of a change in the gene pool.