Let's answer these questions once and for all. As we know, any woman, noble or commoner, must wear black on a visit to the Vatican, except the Catholic sovereigns, regnants or consorts, whose style of Majesty/Highness Catholic has been conferred by the Holy See
So who have the "privilège du blanc" is the Queen Paola of Belgium and Queen Fabiola, the Queen Sofia of Spain and the Grand Duchess Maria Teresa of Luxembourg.
Moreover, the claimants to the thrones of ancient Catholic monarchies, though deposed, don't lost this right, allowing the Princess of Savoy, the Duchess of Braganza, the Duchess of Anjou (because only the Orléanist branch keeps the Catholic style) and the Crown Princess Regina of Austria (while she was alive) the use of white garments in papal audiences.
I hope I have been helpfull.