Hi there, this could be the result of a couple of different things. Your profile doesn't say whether you have a colostomy or an ileostomy. If you have an ileostomy, your food doesn't have as much time to digest, and if you eat spinach or other green leafy vegetables, it can come out looking pretty much like it went in. If your output is very loose and your transit time is very short (the time from eating something to it exiting your body) it can also come out green, as the enzymes in your bowel don't have time to break down the bile in your stool, and turn it from green to brown. It can also be caused by some supplements. If your output is always green, I would probably talk to your doctor about it. If it is just happening once in a while, it's probably nothing to worry about. Always good to check though.
Mine is orange when I eat sweet potatoes... green when I eat spinach... brown with beef. I'm an ileostomy and when I have grape juice it's purple. I was told when I had tomato soup and I freaked out due to the red; it wasn't blood. I guess I'm entertaining my toilet!