not sure, but paediatrician saw our DS's green poos and asked if it was normal for him (which it was), and then said it was fine.
I have read the following - who knows which, if any, are correct!
-too much foremilk
-too much milk
-too little milk
-food intolerance
DS had green poos for 2 or 3 months - have only recently changed to yellow, and that was only since he had a fever for a few days