Feces comes in all different colors many times dependent on what one has eaten. Green vegetables and some dyes in foods are notorious for causing green stool. If that isn't the case, then it generally means that the stool has come quickly through the digestive system and hasn't had the opportunity to pick up the pigment that turns it brown. This is especially common when someone has a touch of gastroenteritis and diarrhea, with the green being from yellowish-green to bright green. If the diarrhea doesn't clear within a few days, you should see your physician for a stool test to make sure there isn't a parasite involved, but gastroenteritis (sometimes erroneously called "stomach flu") is more likely.