In the Savannah, there are many tan and dark rodents. The survival of these organisms will become a quest, escaping from the eagle up above. The eagle will only identify what is noticeable in the brown dirt.
The darker mouse has produced plenty more of the same kind, causing the eagle to eat more. If they produce too much, there's a chance that most of them can't survive in this are of land.
The darker mouse has lower chance or survival based on its fur color because the eagle can only see colors that stand out in the brown sand. The brown animal would be safer.
The eagle will eat the more noticeable organisms. Causing the brown animals to live longer than the darker mice. The gene pool will gradually change over time.