Bowel movements are generally brown in color , because of a combination of bile and bilirubin that is derived from dead red blood cells.
The food you eat typically takes three days from the time you eat it until it finishes its journey in your toilet, If it takes a shorter time, the result may be greener stool because green is one of the first colors in the rainbow of the digestive process.
Yellowing of feces can be caused by an infection known as Giardiasis, or a condition known as Gilbert's Syndrome. This condition is characterized by jaundice and hyperbilirubinemia.
If stool is black, it can mean that you are bleeding internally, possibly as a result of an ulcer or cancer, However, black stools are common when taking a vitamin that contains iron or medications that contain bismuth subsalicylate. If it is red, it is due to the presence of undigested blood, either from lower in the digestive tract, or from a more active source in the upper digestive tract.
Finally a very light-colored stool can indicate a block in the flow of bile or liver disease.