Hatchling and juvenile green turtles are omnivores, taking jellyfish, small molluscs and floating invertebrates in open water. As they mature and settle onto coastal pastures, the diet shifts to seagrass and macroalgae, and the digestive tract lengthens to process plant material with the help of gut microbes.
That shift shows up in the tissue. Plant pigments accumulate in body fat and give it a green cast, which cooks and whalers noticed long before biologists explained it. The grazing also shapes the meadows: green turtles crop the same seagrass plots repeatedly, keeping the blades short, young and higher in nitrogen.
The common error is to expect a green shell. Carapace colour runs from olive through dark brown to nearly black, and varies between populations. Adults typically reach around a metre in straight carapace length and weigh in the region of 100 to 190 kilograms.