I don't know much about the subject but it appears that in all animals (that I know of) its brown, why?
Stercobilin Please Register or Log in to view the hidden image! It's the colour of a chemical that is left when old red blood cells are disposed of. I'm supposed to know this process well (I'm studying medicine), but I confess I had to check some details on Wikipedia. In the spleen: Old red blood cells are chopped up, their hemoglobin broken down, the iron salvaged, and the remains sent to the liver as bilirubin. In the liver: The bilirubin is conjugated (made soluble) and excreted into the intestines with bile. In the intestines: Bacteria change bilirubin to urobilinogen. Some urobilinogen is reabsorbed into blood and filtered by the kidneys. Some urobilinogen is further changed to stercobilin, which is poo brown. In the bladder: Urobilinogen changes to urobilin, which is urine yellow. Cool! Trivia: biliverdin is another chemical in between hemoglobin and bilirubin. It's green, and you can see sometimes see it in bruises. If you're liver is having issues, lots of bilirubin hangs around in your blood instead of being excreted. This can make your skin and eyes yellow (jaundice), your urine dark, and your poo very pale.
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.
You don't see poo when diving because it gets washed away. Try keeping a fish in a tank and you'll see plenty of poo.
