Your bowel movements (poop) are dependent on your food intake, health, and diet. Health affects its viscosity as well as color, and bad health or medical conditions can lead to dark stools.
[Green] may suggest that the intestines didn't break down the chemicals (because of diarrhea), or diet consisting of greens (vegetables).
[Red] stools should be taken with caution, and you should contact your doctor for a check-up.
[Black] stools can be a symptom of a disease, or the side-affect of a medication.
[Yellow] stools may be caused by a lack of bile salts( I'm not that sure, go on google if you are confused)