Most of the times, the colour of the stool changes because of the foods you eat. Similarly, eating a lot of green vegetables can make your stool green in colour. Although the colour of the stool can signal health conditions, you need to first focus on your diet and find out which food is causing a change in your poop colour. Here are some of the foods that cause dark stools. Junk Foods Eating a lot of junk food results in a change int eh colour of the poop. If you eat a lot of junk food, your stool colour changes to dark brown or black in colour. Junk food takes time to break down and can cause weight gain as well. Dark Vegetables From green leafy vegetables to the dark beetroot, foods ranging in dark colours can bring a change in the colour of your poop. If you drink too much coffee or have caffeine rich foods, then your stool will harden as well.