That's a great question and one everyone has thought about at some point. Stool color is determined by a combination of bile, what you eat that isn't absorbed, and how fast or slowly the intestinal stream moves. Bile is yellow/orange and it changes color as it passes in the intestine - to darker colors of brown. Lighter-colored stool are usually from fast transit. Food that isn't absorbed can also color the stool, such as green from certain veggies.
The only colors to be concerned about are pitch black (not just very dark brown), maroon and red.
If you think you are bleeding, it is always more informative to tell your doctor the color of the stool, rather than just describing it as 'bloody'. As red blood travels through the intestine it can turn black by oxidation from contacting stomach acid, or just by traveling through the length of the intestine. So it usually represents upper intestinal bleeding. Maroon color is usually lower bleeding, and bright red indicates bleeding from the lowest location or a very fast transit. This information is important for your doctor to begin the diagnosis.