American Academy of Pediatrics says it well:
“When your baby starts eating solid foods, his stools will become more solid and variable in color. Because of the added sugars and fats, they will have a much stronger odor too. Peas and other green vegetables may turn the stool a deep-green color; beets may make it red. (Beets sometimes make urine red as well.) If your baby's meals are not strained, his stools may contain undigested pieces of food, especially hulls of peas or corn, and the skin of tomatoes or other vegetables. All of this is normal.
Your baby's digestive system is still immature and needs time before it can fully process these new foods. If the stools are extremely loose, watery, or full of mucus, however, it may mean the digestive tract is irritated. In this case, reduce the amount of solids and introduce them more slowly. If the stools continue to be loose, watery, or full of mucus, consult your child's doctor to find the reason.”