Green stool can also be a symptom of various intestinal disorders that interfere with the normal digestion process, such as Salmonella food poisoning. As food passes through the digestive system, a yellow-green fluid called bile that helps digest food changes color, resulting in a stool that is light to dark brown. However, when an infection such as Salmonella causes diarrhea, food and feces pass through the digestive tract quickly before changing to a brown color.