Normal variation in stool color: In infants, especially those who are breastfed, green stools can be a normal variation due to the foremilk being rich in lactose, which can cause rapid transit through the gut and result in greener stools.

Foremilk/hindmilk imbalance: An imbalance where the infant gets too much foremilk (which is richer in lactose) and not enough hindmilk (which is richer in fat) can lead to green, frothy stools.
Introduction of solids or new foods: If solids or new foods have been introduced into the infant's diet, this could alter the stool color to green.
Viral gastroenteritis: Although less common in exclusively breastfed infants, viral infections can cause changes in stool color, including green.

Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia. It's a surgical emergency and can present with changes in stool, including green or bloody stools.
Hirschsprung's disease: A congenital condition characterized by the absence of ganglion cells in the distal bowel, leading to obstruction. Early signs can include changes in stool color or consistency.
Infectious diarrhea (bacterial): Bacterial infections like Salmonella or E. coli can cause severe diarrhea, which may be green in color, and can lead to dehydration and other complications if not promptly treated.

Galactosemia: A genetic disorder that affects how the body processes a type of sugar called galactose. It can lead to green, frothy stools among other symptoms.
Congenital infections: Certain congenital infections, though rare, can cause gastrointestinal symptoms, including changes in stool color.
Gastrointestinal anomalies: Anomalies such as intestinal atresias or stenosis can cause obstructive symptoms, including changes in stool color, though these are typically identified shortly after birth.