These turtles live on a dry, river land. The birds from above spot the turtles from high above to see when would be a good time to eat them.
More turtles are born on the land and there isn't enough space and food for all of them to survive. Some of the turtles are being spotted by the bird's and being eaten.
Not all turtles are green. Some turtles are born with different colors. All the turtles are trying to stay alive and not get eaten by the birds.
The bright colored turtles really stand out from the rest turtles and where they are. That make's the birds easier for them to see. The green turtles blend in with the environment, making them easier to hide from the birds.
Because their color is an adaptation on the land, green turtles are better at avoiding the birds. Green turtles are eaten less and survive more.
Only the surviving turtles get a chance to reproduce because most of those turtles are green. So they pass on their green color trait to their offspring. Therefore, a greater percentage of turtles are green in the next generation.