Regarding the protocol for women who are granted an audience with the Pope, only Catholic queens may wear white - a white dress or suit with a white head covering (often a mantilla). All other women are required to wear black with a black head covering.
I think the white rule, however, also includes princesses of the House of Savoy and the Duchess of Luxembourg. The current head of the House is a bad egg and probably would not be received, but he has three sisters.