Green stool usually occurs when food moves through the large intestine at a pace too fast to give the bile time to break it down completely, explains Mayo Clinic. Diarrhea sometimes causes food to move too quickly through the intestines.
Additional causes for green stool include a diet filled with green leafy vegetables, according to Mayo Clinic. Eating foods that contain green food coloring also turns stool green; this includes food such as ice pops and drink mixes. Taking supplemental iron is an additional cause for greenish-colored stools.
Stool colors vary widely, notes Mayo Clinic. While brown is a common color for stool, green is also considered normal. Diet and bile have a lot to do with the color of stool. When there is a lack of bile in the stool, the stool may be white, clay or other light colors, all of which may indicate a bile-duct obstruction. However, light-colored stools also occur as a result of the use of bismuth subsalicylate or other drugs for treating diarrhea.
Excess fat in the stool can cause a yellow stool color and may indicate the presence of celiac disease, states Mayo Clinic. Black stools indicate bleeding in the upper gastrointestinal tract but also result from iron supplementation.