What happens when a turtle shell turns red?

The name "green turtle" actually comes from the color of the fat.

The fat is green due to the turtles' diet; they eat mostly sea grasses (such as turtle grass and manatee grass), and algae that commonly grow in lagoons. Their external colors change with age; colors on the turtles include various shades of black (prominent in hatchlings), brown, olive, and yellow.