What reason green stool for 3 month baby?

Green is OK!: Baby poop is lots of different colors and can change from day to day for any baby. Amount of bile output varies. One day brown, one day yellow, one green. The worrisome colors are dark black after the first few days of life (cuz your first poop is blackish meconium), red poop or pure white poop. as his digestive tract matures, consistency will also vary. And yes, baby poop stinks!

Faster transit time: when stool comes through faster, it stays green instead of yellow; doesn't have toime to convert. Could be something mom is eating (breast feeding) or iron vitamins; if baby is having diarrhea, green could be sign of infection

Transition: The color of poop depends on the status of the color additive. Bile added to the poop starts out deep green and transitions through lighter shades, then yellows then browns depending on the action of germs on the bile. The faster it moves through, the greener, the slower, the more likely to be brown. Green color by itself is not a sign of disease.