Green stool can indicate that food traveled through the intestine too quickly for bile to break it down or that the person ate a lot of green vegetables, according to MedicineNet. Stool that is yellow and greasy may indicate fat digestion problems, and yellow coloration alone suggests a gastrointestinal infection.
Cystic fibrosis and celiac disease are examples of conditions that can cause an inability of the intestine to digest fat, resulting in yellow, greasy stool, notes MedicineNet. Poor fat digestion can also occur when the pancreas fails to produce the necessary digestive enzymes.
Unexplained black stool is a medical emergency because it may indicate internal bleeding in the gastrointestinal tract, duodenum or stomach, warns MedicineNet. Another cause of black stool is swallowed blood in the case of mouth injuries, oral surgery or nosebleeds. Stool can also be black due to certain foods, such as beets; supplements, such as iron; or medications, such as bismuth.
Bright-red stool can also indicate the presence of blood in the stool, most commonly from hemorrhoids, as MedicineNet explains. Other causes of bright-red stool include bleeding from inflammatory bowel diseases, arteriovenous malformations and tumors. Eating foods containing red food coloring can also cause stool to be bright red. White or pale stool suggests diseases of the biliary tract or liver.