I've had that same issue from time to time and found that if I eat black licorice or a blue slushie, it will give me green stools.
A few things: too much bile in the stool (could indicate a future upcoming gallbladder issue a few years down the line), decreased colonic transit time, so that the stool doesn't have time to change colors. Some food colorings and certain vegetables will cause stools to be green. Really, green is about the most un-worrysome color for stools to turn. If you're worried, talk to a doc, but you're probably just fine.