Ah yes, the color of your stools. This is easily one of the more important mysteries facing modern medicine today. Fortunately with the help of technology and thousands of the best and brightest scientists, we now have answers to this vital question. Not surprisingly, the answer is not nearly as complicated as we once thought.
In medieval times it was thought that the color of your stool was related to the degree of demonic possession in your large intestine. Darker stools meant more demons crawling around in your bowels. Of course, we now know this to be nonsense - modern medicine has proven that darker stools are clearly related to alien abduction experiments, not demonic possession. Although it's easy to see how the two could be confused. Further evidence may support the somewhat controversial hypothesis that demonic possession and alien abductions are actually one in the same. Stay tuned.
OK, now for the serious answer.
The color of your stool is largely dependent on what you've eaten recently. After all, your stools are mostly undigested waste of food that your body has no use for. Just as your urine is the way your body gets rid of liquid waste, your bowel movements are the way to rid your system of unneeded solids.
Excessive fat in your bowels will often make your stools lighter in color. Fat will also make your stool more likely to float, a valuable thing to know if you frequent public swimming pools.
Although I've never done a formal experiment, if you were to eat five pounds of carrots every day for three or four days, I imagine you would begin to see a distinctive orange tinge to your stool within a short time. Just as you are what you eat, your stool is what you eat as well - in fact, even more so.
One of the major constituents of stool color is the result of breakdown of red blood cells. Red blood cells have a lifespan of approximately four months. After this time, they become damaged a point where they are filtered out of your arteries by the spleen. The products of that processing include a chemical known as bilirubin. Bilirubin is a pigmented chemical that is, not surprisingly, reddish brown. Bilirubin is excreted in to your stool, giving it that lovely brown color we all know and love.
Of course, dead red blood cells aren't the only kind that can make their way in to your large intestine. Bleeding in to the bowels can produce distinct red coloration to your stools. Bright red blood in the stool is a sign of active bleeding and should be seen and evaluated by a doctor as soon as possible. Darker red blood is often associated with bleeding up higher in the GI system, such as a peptic ulcer. This too can be quite serious and should be evaluated by your doctor.
Now, at this point you may be saying to yourself, "Yeah, that's all very nice... but what about the smell?"
I'm glad you asked.
Simply put, the smell associated with your stool is derived from nothing less that millions of microscopic bacteria. It seems that bacteria love your undigested food and use it as a source of energy. Your bowels tolerate these bacteria as they help further breakdown your stools past the point where you can do it with your natural metabolism. And as an added bonus, the waste products of those hard working bacteria come in the form of some very aromatically insulting chemicals.