Consuming leafy green vegetables, iron supplements and green food coloring, typically present in flavored ice pops and drink mixes, may cause dark-green stools, explains Mayo Clinic. On occasions when food moves through the large intestine too rapidly, bile doesn't have time to break down competely, causing dark-green stool.
The foods an individual consumes and the amount of bile, which is a yellow-green, fat-digesting liquid, in the stool usually determine stool color, notes Mayo Clinic. Enzymes chemically alter bile pigments travelling through the gastrointestinal tract, changing the pigments from green to brown. All shades of green are within normal ranges. Individuals should seek medical attention when stool is black or red, as this may signal bleeding or a disease in the body.