Apparently it is not abnormal for the child to have green stools occasionally, like once in a few days, but if it becomes a common occurence then this is when it can be signalling something else is going on.
- Breastfeeding and green stools..
- Formula fed babes and green stools..
Ref: Pediatrics. 1995 Jan;95(1):50-4.
It seems as though green stools come about from the faster passage through the digestive system. If the time of passage is slower, the poo is more mustard colour. This way it allows time for the movement of nutrients in and out of the intestine.
In pre-term infants, one study found that the colour and consistency of stool vaired according to the different pre-term formulas given.
Turk J Pediatr. 2000 Apr-Jun;42(2):138-44.