Can you actually quote that statement here? What exactly does it say? Is it true then that the Prince of Monaco was given the very old and special status of, let's name it, Most Catholic monarch (Catholicissimus; the titles vary on certain monarchs)? Because historically, the privilege du blanc (not eiglibility to wear white while visiting the Pope in general) was accorded to the consorts of the said Catholic monarchs (and by Catholic I mean not all those of Catholic faith) and the monarchs themselves if they were females, of course.
I totally agree with you that today the ancient privilege du blanc is not applicable anymore. Everyone, especially Charlese and other women of her status, can absolutely wear white in the Vatican or any other colour they feel is appropriate to wear in front of the Pope. But if we are talking about the privilege we can't change the ancient rules. The Pope can, of cource, but do you have any proof that he did so? Just because the Princess of Monaco wore white once in the Vatican and that its press officers stated that it was not inappropriate for her to do so, means nothing. Many other high-profile women before her without the privilege du blanc wore dresses in other than black colour while visiting the Pope. Cherie Blair's faux pas from several years ago is well-known. I also remind seeing a picture of Danuta Walesowa, the wife of former Polish President Lech Walesa, wearing white during an audience with Pope John Paul II. Gorbachov's wife wore red. Both female Presidents of the Irish Republic did not wear black while visiting the Pope.