Should be mustard like and yellow. My ds has very runny poop (TMI) - any time someone new watches him they tell us he has diarrhea and we have to explain that no, that is just how he is. Our dr. told us it can very from lo to lo.
If ds has an upset stomach or is getting to much of the foremilk, then it is green. May I ask what are you noticing as the change?