How does a newborn change?

A baby has been born, and we expect him to resemble the beautiful, rosy infants seen in advertisements. But he sometimes appears red, then suddenly yellow, with rashes or peeling skin. On top of that, his weight is unstable, and his stools are unpredictable— is the baby healthy? Yes, he is, and all these changes are known as transitional states. Where do they come from, how do they appear, and what should we do about them?