Your stool, for example, can span a rainbow of brown, maroon, and green tints and still be considered within healthy bounds. Much of its coloring depends on the concentrations of bile in your system. Produced by the liver and excreted into the small intestines, bile contains cholesterol, bile salts to help digest fats, and waste products such as bilirubin. As the bile pigments are broken down by stomach enzymes, they tend to change from yellow-green to brown. However, certain colors may also indicate a serious intestinal conditions—and potentially even some forms of cancer:
Green: Overtly green stool can be caused by a number of factors. It may indicate that food is passing through your system too quickly (read: fast-food induced diarrhea), preventing the bile pigments from being sufficiently broken down. It could also be caused by consuming large amounts of leafy green vegetables, excessive amounts of artificial food coloring, or even licorice candy produced with anise oil rather than actual licorice herb. Some people have a sensitivity to Anise oil and may develop loose green stool after consuming it. Or, if you are on an iron supplement regimen (often used to treat Crohn's disease and as a supplemental other ADHD treatments), bright green poops are a potential side effect, as are constipation and diarrhea, so let your doctor know if any occur.
White: Stool that is clay-colored or white is caused by a lack of bile in your stool, potentially caused by a bile duct obstruction. If your biliary system is blocked—by, say a gallstone, enlarged lymph nodes in the porta hepatis, or inflammation and scarring of the bile ducts—bile will back up into the liver, causing not only white stool but abdominal pain, dark urine, and jaundice (yellowing of the skins and eyes) as well.
Yellow: Yellow stool that is also consistently greasy and smells of sour eggs (due to the presence of hydrogen sulfide) may be caused by high levels of fat in the stool that have not been broken down by the bile. This is one symptom of Celiac disease, so if you see this floating in the toilet, definitely talk to your your doctor.
Black: Black stool is a surprisingly common side effect and can be caused from anything from a night of binging on black licorice and Guinness, to your iron supplement regimen, to ingesting large amounts of bismuth subsalicylate aka Pepto-Bismol. This happens when the bismuth subsalicylate combines with trace amounts of sulphur in your saliva to form bismuth sulfide, a highly insoluble black salt that can stain the tongue and stool jet black. Luckily, it is a temporary condition. However, black stool may also be an indicator of bleeding in the upper gastrointestinal tract, caused potentially by an ulcer or tumor. This bloody stool will often take on a tar-like appearance and smell horrible, so if you suddenly poop a poop that's black and sticky and smells like something crawled up there to die but haven't spent the last 16 hours at the pub, see your doctor.
Bright Red: Another symptom that could be deadly or could be nothing is bright red stool. Red stool is fairly common, often instigated by natural and artificial food colorings found in beets, cranberries, tomato juice, red gelatin, and drink mixes (ie Kool-Aid). But stool with bright red spotting or free floating bands of fresh blood are a sign of bleeding in the lower intestinal tract brought about by a case of hemorrhoids.
Blue: If your poo is blue, there's a good chance you'll already know why. This is an extremely rare side effect of consuming ferric ferrocyanide—better known as Prussian blue, an insoluble bright blue pigment used in the treatment of heavy metal (radiation, cesium, and thallium) poisoning. Blue poo can also be caused by guzzling large quantities of blue curaçao and grape soda.
Silver: Silver poop is both very possible and a very bad indicator of your intestinal health. If your stool has the same color as a tarnished candlestick, it could indicate that you are suffering from both a biliary system blockage and upper intestinal bleeding, Basically white stool caused by a lack of bile mixes with gastrointestinal blood, which stains it the same color as aluminum spray paint. So if your poo looks like something the Tinman would pass, hustle yourself down the yellow brick road to the Wizard of ER.
Purple: Congratulations, you have porphyria.The article also discusses the texture of various stools, and what the texture may indicate--with illustrations! A grotesque topic, I know, but still something helpful to know for health/first-aid purposes.