Massimo Gatto traces the history of the most conspicuous retired red shoes in Rome. They were once a symbol of wealth and power:

Those red shoes, for example—which the pontifex emeritus has now given up in favor of a more ordinary brown pair from Mexico—may symbolize the blood of Christian martyrs. But when red shoes were the height of fashion in Etruscan Rome, that is, five hundred years before the birth of Jesus, they designated the wearer as an aristocrat, someone who could afford leather that had been colored with the most expensive dye in the Mediterranean, Phoenician “purple”—which was actually scarlet red. (It was produced by scoring the bodies of molluscs and ranged in color from blue to red, with red the most prized shade). The leather itself came not from kangaroos, of course, but from the Chianina cattle, who came to Italy together with the Etruscans and provided the ancestral form of Florentine beefsteak.