I've had green and red-green is spinach or other green leafies, red is beets.
Horse poo is green. Coyote poo is white (chewed bones). Rabbit poo is almost black. Insectivorous bird poo is black and shiny with chitin.
If you puree a bunch of food, it usually turns brown. Oxidization also turns things brown. So I'm wildly guessing these are related factors.