When your baby starts solid food, at about 4 to 6 months, you'll notice new colors in his diaper. For example, green foods (pureed peas, spinach, beans) can produce green-colored stools, and orange foods (carrots, squash), can produce orange to yellow stools. Don't be alarmed -- orange, yellow, green, or brown stools can all be normal.
Baby food tends to make the stools of breastfed infants firmer and the stools of formula-fed infants softer, but in either case the stools will probably smell worse. Normally, when you introduce solids, you'll start your baby on rice cereal and foods such as bananas and applesauce, all of which tend to firm up stools. In most cases this poses no significant problem for your baby, but sometimes she can develop constipation. To combat constipation, balance your infant's diet with foods that soften stools, such as pears, peaches, plums, apricots, peas, and prunes.
Red-colored stools can often be explained by something your child ate or drank, such as fruit punch or tomatoes. But occasionally, a red spot can be blood. This is a frequent cause of alarm for parents, but in most cases blood in the stool is caused by nothing more than a fissure -- a tiny tear along the rectum that will heal by itself. Fissures can result from a parent's being a little too aggressive with a rectal thermometer or from a recent bout of constipation or diarrhea. However, blood in baby's stool can also be a sign of an allergy to cow's milk formula. Other signs that your child may have an allergy to formula include diarrhea, vomiting, skin rashes, and poor weight gain. Let your doctor know if your baby experiences any of these symptoms. If your baby has an allergy, she'll probably need to get switched to a hypoallergenic formula.
Once your child is 9 months to a year old, her menu expands to table food. Age 1 is also when your baby can start drinking whole milk. During this time, your baby's stools start to take on the familiar form and more consistent brown color of an older child's or adult's stool. And because toddlers don't always chew their food well, it's not uncommon to find pieces of undigested food, such as corn or peas, in their stool.