Hmm, do all Catholic sovereigns have that privilege, or only her Most Catholic Majesty, the Queen of Spain?
Absolutely beautiful photo, her Protestantism disregarded...
Besides Her is Mgr Jacques Martin, later cardinal : a wonderful man I met so many times when I was assigned to our Emassy to the Holy See.
When Mgr Martin was made a cardinal, I offered him a red zucchetto (from Gammarelli, of course), the day after the announcement was made. He lived with his sister near St Peter's basilica : he gave me his purple "calotte" and I still keep it preciously.
The privilege of white is now enjoyed by all Catholic Princesses (term more inclusive than merely Queens (e.g. Spain, Belgium), since the Granduchess of Luxembourg also wears white now in the Vatican).