When they are born, babies have a thick dark-green or black substance in their intestines, called meconium. The first few days involve them passing this substance; this needs to happen before normal digestion can take place. When all the meconium has passed, stools will be a yellow-green colour.
Since your baby is breastfed, it is expected that stools passed are yellow, or mustard in colour. The consistency ought to be soft and slightly runny. Initially, the stools should have seed-like particles. The consistency and colour remains more or less the same until solid food is introduced. Formula fed babies produce stools that are mustard-yellow in colour, but firmer and drier than those of breastfed babies. The consistency should not be firmer than peanut butter though.
If there are very hard or dry stools, then it is a sign that your baby is not getting enough fluid, or that there is fluid loss due to fever, sickness, or environmental heat. You should consult with your G.P. or paediatrician.
Once solid foods are part of the normal diet, hard stools show that there is too much constipating food being eaten. Cow's milk is therefore not allowed for children under 12 months, since their digestive systems cannot handle it.