The population of the mice has started with 100% of only one type of allele, and it helps it blend in perfectly with its backgrounds whenever a predator is around.
In the mice population something changed which was caused by mutation in the population. Two light brown mice, but they don't blend in which makes it harder for them to survive.
A snakes appears due to the change of mutation that happened in the mice's population, and the cause of that it made it way easy for the predator to peep mutated mice, giving its variation.
The mutated mice can now blend in with its background, it can camouflage and not be get eaten by a predator easily. Due to all the variation in the original allele in the mice, there are more vulnerable to the predator .
The mice are now 100% surviving from it's population, because the original allele faded away, and now the color blends in, and it's hard for predators to find mice if it's camouflaging with it's background.
The predator was out for food, but cant see the mice since they camouflage their background, and a hawk peeps the snake since the snake's allele doesn't blend i with it's background, the snake doesn't survive.