Dark poop can be liver function or too much protein. It also can be low digestion, and adding water to the food will help alot.
If it goes yellow- that is really liver- but I thinik what you are saying is a gassy stool= added water to the food beforehand - ( warm water is best) this should correct this. Also adding pedialtye for normal bowls just a 1/2 cup to the bowl would do it..