Report Abuse. Brown. Feces are mostly shades of brown or yellow because they are also made up of an orange-yellow substance called bilirubin that is present throughout the body and particularly in the intestine. When iron combines with bilirubin, it gives it a brown color.eport Abuse. Brown. Feces are mostly shades of brown or yellow because they are also made up of an orange-yellow substance called bilirubin that is present throughout the body and particularly in the intestine. When iron combines with bilirubin, it gives it a brown color.