Bile, mucous and the colors of the foods you eat can all determine the color of your stool. Brown is considered normal, but if you eat a a diet high in leafy green vegetables or foods with green food coloring, your bowel movements may assume a green shade. If you notice that your stool is bright red or a tarry black, you could be passing blood and should see your doctor immediately for testing. Bright red stools usually means blood is coming from your large intestine or rectum and black means the bleeding stems from your stomach, such as when you have a bleeding ulcer. It is possible that your red or black bowel movements are caused by your diet; colorful foods like beets, tomato soup and red gelatin desserts can color your stools red, and iron supplements can make your stool black. White-colored stools are abnormal and should be checked out immediately. A light-colored stool may indicate a bile ducts blockage, gallstones or metabolic disorders.