I heard that green was a cold in the stomach.
Green stool usualy means rapid intestinal transit - bile, which is originally green does not have time to be changed by intestinal bacteria into an usual brown color. Next cause could be - spinach :) and such...or a parasite giardia, maybe Clostridium difficile. If you think it's not for food, have stool test for parasites and Clostridium difficile toxin.