Stool color is generally influenced by what you eat as well as by the amount of bile — a yellow-green fluid that digests fats — in your stool. As bile pigments travel through your gastrointestinal tract, they are chemically altered by enzymes — changing the pigments from green to brown.
Consult your doctor if you're concerned about your stool color. If your stool is bright red or black — which may indicate the presence of blood — seek prompt medical attention.