A zucchetto is a spherical skullcap worn by members of the clergy. A priest's is black, the bishop's is purple, a cardinal's is scarlet, and the pope's is white. But where did this come from? It is thought to have originated in the early Middle Ages, if not earlier, as a way to help keep their heads warm.
All of the outfits, if you will, have some such meaning or historical reference, even if they have no apparent significance now. For instance, usually our clergy do not need help keeping their heads warm now with the advent of heaters!