Changes in stool (feces) color are often harmless and reflect dietary influences. The normal brown color of stool occurs due to the presence of bilirubin. Bilirubin is formed as a breakdown product of hemoglobin (from red blood cells) in the liver and is secreted into the bile, which enters the intestines. If the intestinal contents travel at a normal speed, chemical changes in bilirubin produce stool that is light to dark brown. The stool may appear green if the intestinal contents pass through the bowel more rapidly.

Changes in the color of stool are common and are generally not significant if the changes are noted from one stool to the next and are not persistent. However, changes in the color of stool can also be caused by a number of minor to serious medical conditions and can occur with certain medications. Changes in stool color that are persistent and do not occur in only one stool may signal the presence of a medical condition and should be evaluated.

Certain persistent changes in stool color are characteristic for specific conditions:

• black, foul-smelling stool: intestinal bleeding (typically from the stomach and upper small intestine) due to ulcers, tumors; ingestion of iron or bismuth
• maroon stool: intestinal bleeding (from the middle intestine or proximal colon) due to ulcers, tumors, Crohn's disease, ulcerative colitis
• clay-colored stool: lack of bile due to blockage of the main bile duct
• pale yellow, greasy, foul-smelling stool: malabsorption of fat due to pancreatic insufficiency, as seen with pancreatitis, pancreatic cancer, cystic fibrosis, celiac disease

Source: http://www.rxlist.com