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Green: 1  Green stools are always normal, but they can be mistaken for black stools. 2  Bile. Most dark green stools are caused by bile. 3  Green stools are more common in formula fed than breastfed infants. It can be normal with both.  Green stools are more common with diarrhea. This is due to a fast transit time through the gut.
Iron supplements and bismuth-based medications, for example, can darken your stools. Sometimes, serious blood and circulation abnormalities in your digestive system can cause black, tarry stools. 1  Bowel ischemia is a reduction of blood flow to the intestines.  Vascular malformation is misshapen veins.