There are many different reasons why our bowel movements may come out in different colors from time to time. The majority of the time these color changes are perfectly normal and harmless, although there are some signs to look out for that could mean that you require medical attention.
As we all know the color of bowel movements under usual circumstances is a darker shade of brown, but there are actually a surprising array of colors that can be present due to different factors, such as black, red, green and even yellow. Hundreds of years ago in fact the color of a persons stool and urine samples were considered one of the most important tools in medical diagnosis for a wide array of illnesses, and as a means for assessing the general health of a person as well.
Most of the time of course the color of our bowel movements are affected by what we have recently eaten, and the color that is produced will often reflect the color of the food that you have eaten. For example if you have eaten something that is red such as the flesh of certain fruits, then the stools produced afterward may be red. Similarly if you eat something that cannot be digested properly by the body, or that you might have an intolerance to, then this can also affect the color produced.
If there is a yellowish color to the bowel movement then this is usually the result of there being a higher then normal concentration of the bacteria that break down dead red blood cells. There are always a small amount of red blood cells present in any stool, but a yellowish tinge means that this is an above normal level and if persistent should be diagnosed by a doctor.
If the stool is either very dark towards black, or just very dark red then it suggests that there is a lot of blood present in the stool. Brightly colored blood associated with pain is usually a sign of hemorrhoids which aren't usually too much of a concern, and will usually be apparent to the sufferer anyway. However the darker the blood is suggests how far up the digestive tract it has come from (darker being further up). This can be the result of things such as ulcers inside the stomach or intestines or from some internal bleeding, which can be caused by a number of factors.
Lastly a greenish color is a sign that the food you have eaten has been processed too fast by the body and that the chemicals in the liver that usually break down lipids are still present in the stool. This can be as a reaction to the food that you have eaten of from general diarrhea caused by an infection or stomach bug.
It is always important to note any changes in your regular diet and any changes in your bowel movements that this brings with it. If any symptoms or color changes are persistent or painful then the best thing to do is to consult a doctor immediately.