According to livescience.com:
The complex digestion process ensures that almost no useful energy goes unused. The average bowel movement is three parts water to one part solid matter. Bacteria make up 30 percent of the solid stuff. The same goes for indigestible foods like cellulose and extra fiber. The remaining 40 percent contains various inorganic wastes, fats and used-up body substances like red blood cells, which are released from the liver in an orange-brown compound called bilirubin.
Bilirubin mixes with another liver product, yellowish bile, to give poo its distinctive hue.
So know you know!