My 3 months old baby having a dark green poops for almost 2 months, now his having a dark gray sticky poops.i'm not changing his formula since then.

Normal variation: The main agent adding color to the poo is the material made by the liver called bile. It starts out a dark green and goes thru the yellows to brown depending on the influences of various factors in the intestines. Rapid transit thru the system, as may occur with diarrhea or breast feeding may have a greenish yellow or yellow appearance. The slower, more likely browner.