Stool color is influenced by what you eat as well as by the amount of bile (a yellow-green fluid that helps digest fats) in the stool. As bile pigments travel through your gastrointestinal tract, enzymes chemically alter the pigments from green to brown. Changes in the color of your stool don’t have to indicate a disorder. For example, food can have a huge impact on the color of your stool: if you eat black licorice, iron pills, bismuth medicines, or blueberries, you may have black stools. Beets and tomatoes can influence stool color and cause it to appear reddish.
Bloody Or Tarry Stool
Bloody stools often indicate an injury or disorder in the digestive tract. There is a big difference between stool filled with fresh blood and stool that is black due to blood in it. Melena is a term used to describe black, tarry, and foul-smelling stools. 
Hematochezia refers to red or maroon-colored stools. Blood in the stool can originate from anywhere along the digestive tract, from the mouth to the anus.
A black stool usually means that blood is coming from the upper part of the gastrointestinal tract, which includes the esophagus, stomach, or the first section of the small intestine. The reason that blood appears like tar is that the body exposed it to digestive juices.
Common causes of upper GI bleeding are :
- Bleeding in the stomach or duodenal ulcer
- Esophageal varices
- A tear in the esophagus from violent vomiting
- Trauma or foreign body
- A lack of proper blood flow to the intestines — ischemia
- Vascular malformation
Maroon-colored stools or fresh red blood in the stool usually suggests that the blood is coming from the lower part of the GI tract, which includes the large bowel and rectum.
Hemorrhoids and diverticulitis are the most common causes of lower gastrointestinal bleeding.
Other common causes of lower GI bleeding are :
- Anal fissures
- Intestinal infection
- Vascular malformation
- Inflammatory bowel disease
- Colon polyps or colon cancer
- Trauma or foreign body
A variety of things can also cause rectal bleeding :
- Diverticular bleeding
- Colon polyps
- Peptic ulcers
- Inflammatory bowel disease
- Lack of blood supply to the bowel
- Colorectal cancer
Bloody Bowel Movements: Diagnosis
Fecal occult blood test
Sometimes the blood may be present in such small amounts that you cannot see it. However, it is detectable by a chemical test of the stool called a fecal occult blood test.
Endoscopy and x-rays
If there is enough blood to change the appearance of your stools, the doctor will want to know the exact color to estimate the site of bleeding. Sometimes an endoscopy or special x-ray studies are needed. In these cases, your doctor can test the stool with a chemical to rule out the presence of blood.
- Blood studies, including a CBC and blood differential
- Barium studies
- Stool culture
- Tests for the presence of Helicobacter pylori infection
Prevention Of GI-bleeding
- You should eat vegetables and foods rich in natural fiber and low in saturated fat because these foods may prevent constipation, hemorrhoids, diverticulosis, and colon cancer.
- You should avoid the prolonged or excessive use of anti-inflammatory drugs like ibuprofen, naproxen, and aspirin because they can irritate the stomach and cause ulcers.
- Significant amounts of alcohol can irritate the lining of the esophagus and stomach.
- Smoking may cause peptic ulcers and cancers of the GI tract.
- Stress is also a possible factor in peptic ulcer disease.
Diarrhea Or Watery Stool
After a meal, the small intestine absorbs fluid and nutrients from the food that you have eaten. Anything that disrupts this process may cause diarrhea. For example :
Viruses are the most common cause of diarrhea. Medicine has proven that an invading virus can damage the lining of the small intestine, disrupt fluid and nutrient absorption. Fortunately, the symptoms usually disappear in approximately three to four days.
Some bacteria which can contaminate food or water can release a toxin that causes intestinal cells to secrete salt and water into the lumen of the bowel. This is more than the capacity of small intestine and colon can handle.
Other inflammatory agents
A parasite or a reaction to medication such as antibiotics can sometimes cause diarrhea. Once you eliminate the parasite or discontinue using an offending antibiotic, diarrhea usually disappears. Caffeine and alcohol stimulate the passage of stool so consuming too much of those may cause the waste to move through small intestine and colon too quickly.
Diarrhea may also be related to an intestinal disorder. Possible causes include irritable bowel syndrome, inflammatory bowel disease, lactose intolerance and celiac disease.
Tips For Preventing Diarrhea
- Drink plenty of fluids
- Gradually add solid foods to your diet
- Avoid anything that may prolong diarrhea and avoid fatty foods, spicy foods and beverages containing caffeine or alcohol
- Don't take antacids containing magnesium
In case you're traveling to developing countries and you suffer from traveler's diarrhea, you can prevent and cure it by being careful about what you eat and drink. Here are a few tips you should follow :
- Use only bottled or purified water for drinking or even brushing your teeth.
- However, if you do use tap water, you will need to boil it or you can use iodine tablets.
- Eat only fully cooked food and served hot.
- You should avoid unpeeled raw fruits and vegetables.
- Avoid unwashed raw fruits and vegetables. Wash raw fruits and vegetables with purified water.
Pale Or Clay-Colored Stools
Jaundice or yellow skin often accompanies clay colored stools. Since it is the bile salts excreted by the liver that give the stool a normal brown color, an obstruction to bile flow out of the liver or liver infections like viral hepatitis may produce clay colored stools. Certain medications, such as large doses of Kaopectate and other anti-diarrhea drugs, may also cause such symptoms. Other causes such as alcoholic hepatitis, biliary cirrhosis, metastatic or benign tumors or cysts of the liver, biliary system, or pancreas, and gallstones. 
Mucus In Stool
Mucus in the stool is generally not an indication of a serious problem as stool normally contains a small amount of mucus. It is a jelly-like substance that intestines make to keep the lining of the colon moist and lubricated. Increased amounts of mucus in stool may occur with diarrhea or constipation .
However, sometimes mucus can indicate an underlying medical condition such as ulcerative colitis or Crohn's disease.
Although it could be a sign of normal eating patterns, stringy stool may also be a cause for concern. The patient could have a parasitic infection.
Smelly Or Greasy stool
Changes in your diet may cause changes in the smell, texture, and color of your stool. Some say a vegetarian diet causes less odor in the stool, while fat seems to cause smellier stools. If someone has particularly greasy, smelly, or floating stools, he or she may have a problem absorbing fat from the gut. The problem may be the pancreas or small bowel.
Silvery White, Gray Or Pale Yellow Stools
If you notice silvery white, gray, or pale yellow stools, a physician should be consulted immediately as disorders involving your liver, gallbladder or pancreas may be the cause and your stools may represent a serious disease.
Constipation is a condition characterized by the repeated painful or difficult passage of hard stool or having a bowel movement only once or twice a week. Eventually, the waste becomes dry and difficult to pass. There are a few common culprits :
- Eating and drinking habits
- Sedentary lifestyle
- Inattention to bowel habits
- Other medical problems such as hypothyroidism and Parkinson's disease.
Self-care For Constipation
Lifestyle changes are often the safest way to manage constipation. To help ease symptoms :
- Drink plenty of fluid.
- Eat more fiber, including flaxseed or flaxseed oil.
- Regular meals could be beneficial in treating constipation.
- Exercise stimulates the intestinal muscles.
- Reduce stress because stress can slow digestion.
- Use laxatives with caution.