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.
Well, never having actually seen the protocol, I could be off a little. It may be Catholic royal ladies that are essentially the political figureheads of their respective countries. For example, if Prince Albert of Monaco were ever to get married, his wife would be the Princess of Monaco and if she were Catholic, she could wear white when seeing the Pope. At least that is how I understand it. Not being of royal lineage nor likely meeting the Pope any time soon, I don't really need to worry about it. ;-)