When your baby's born, the first few days the stool will look dark and tarry, and that's called meconium. Meconium looks almost like a black, maroon colored paste. That will be the first one day to, let's say, three days of stools will look like meconium.
After the meconium is passed, the baby's stool will look somewhat like Dijon mustard. It's yellow and green, and it's seedy and watery. Breast-fed babies have more liquid-y stools; bottle-fed babies have more form stools.
Stool can range in colors from brow to yellow to green to light grey-green. As long as there's no blood or blood streaks in the stool all of that is fine.
If you notice a change in your baby's stool where it becomes very, very hard or very, very mucous-y, you should discuss that with your pediatrician.
One final point is that often parents call because they think their babies have diarrhea. Newborn poop can be very runny. It can look like water with a little bit of form stool inside. That's normal. As long as there's no blood, and as long as it's not only water coming out of the stool, then your baby should be fine.
If there's any change please contact your pediatrician.