Why do feces vary in color so much less than the food they are results of? E.g. you may eat food of all kinds of different colors but the resulting feces will almost always be some shade of brown.
The main reason for the color of our stool (or feces) is the presence of bilirubin, a breakdown product of the hemoglobin. The amount of it causes different variations from yellow to dark brown. When the passage time of the food in the intestine and the amount of bilirubin is constant, the color changes very little. Illnesses like diarrhea, which cause a lower passage time have therefore a ligher color because a lower amount of bilirubin can accumulate.
The color of the stool can also indicate illnesses of the internal organs, white stool (or pale clay like colored) can for example indicate problems with the biliary system (drainage system of the gallbladder, liver and pancreas).