The color of stool can vary dramatically and can also be a clue as to whether various disease states are present.
Normal stool is brown due to its composition: bacteria, water, bile, bilirubin, broken-down red blood cells and indigestible plant matter like cellulose, along with small amounts of protein and fat.
Red stool is most worrisome as it indicates bleeding in the lower gastrointestinal (GI) tract from conditions like hemorrhoids or diverticulosis, or more serious conditions like rectal cancer. Red stool can also be caused by ingesting red food coloring or beets. While it should always be reported, it's not always an ominous sign.
Green stool can occur with rapid transit through the intestines where bile doesn't have a chance to be broken down to its final brown color. Green can also be a sign of Crohn's disease, antibiotic use, ingestion of leafy greens or iron therapy.
Yellow stool can be the result of gallbladder dysfunction which causes improper handling of bile. Infection with giardia lamblia produces a characteristic yellow diarrhea. In addition to causing diarrhea, different types of infection in the GI tract, whether viral, bacterial or parasitic, may cause changes in stool color.
White stool can be a sign of fat malabsorption, as with pancreatitis and pancreatic cancer, but barium used for X-rays can also give the same appearance. Mucus in the stool can give it a whitish appearance and may be due to inflammation or benign conditions like irritable bowel syndrome (IBS).
Black stool should trigger a search for bleeding from the upper part of the GI tract (esophagus, stomach or small intestine), but can also be seen with iron therapy, heavy meat consumption, and bismuth-containing compounds.
Light-appearing clay-colored stools are characteristic of liver disease and decreased bile output, but can also be caused by antacids containing aluminum hydroxide. Vitamins and supplements commonly cause changes in urine color but may also change stool color.