What does the Roman Catholic custom called “the privilege of white” entail?
"Le privilège du blanc" (Italian: "Il privilegio del bianco") is a French term meaning "the privilege of white". It is used as a Catholic custom whereby certain "designated royal women" are permitted to wear a white dress and veil during audiences with the Pope. This is an exception to the long tradition of women wearing black on such occasions.
Protocol for papal audiences has required that ladies wear a long black dress with a high collar and long sleeves, and a black mantilla over the head and shoulders as these colors historically signify piety and humility. Even non-Catholic women of nobility, like the Queen of England, adhere to this dress code. However, Pope Francis seems to have relaxed this tradition.
The privilege is not accorded to all the wives of all Catholic monarchs. As of October 2019, only seven royal women officially qualify for the "privilège du blanc" dress code and are exempt from wearing black. They are: Queen Sofía of Spain; Queen Paola of the Belgians; Queen Mathilde of Belgium; Queen Letizia of Spain; Maria Teresa, Grand Duchess of Luxembourg; the Princess of Monaco by dispensation from Pope Benedict XVI to the House of Grimaldi in 2013 and the Princess of Naples, by dispensation from Pope Pius XI to the House of Savoy in 1929.