What kind of medication can cause green poo?

Lol! There are some cereals that would cause green poo. I had the hardest time figuring it out. I had 5 babies under 5 and they would all develop this kind of scary green poo. I finally figured it out that it was this cereal called Kaboom they were eating. It had red and green colors (must have been the food coloring they added, it was cheap cereal but high in iron). Iron can cause black stools, but I've heard that it can cause green ones too.