The undiscovered colorful mice species are found deep in woods. They are eaten by birds that can spot them from above.
The mice continue to reproduce and more are born than can survive in the woods. There are not enough resources for all of them to live, so some of them will be spotted by the birds and eaten.
Since they are colorful mice, they are all competing to stay hidden from the birds so they don't get eaten.
In the green woods, the blue and purple snails stand out a lot more than the green. The birds from above can see the purple and blue mice more than the green ones cause they match the color of their environment, which makes it easier to hide.
Because their color is an adaption to the woods, green mice are better at avoiding the birds. Green mice are eaten less often than the blue or purple ones.
Only the surviving mice get a chance to reproduce. Because the majority of them are green, they will pass on their green color trait to their offspring. Which means there is a greater percentage that the snails will be green in the next generation.