Since King Felipe and Queen Letizia met with Pope Francis today–their first overseas engagement as King and Queen–I thought it was an appropriate time to discuss the protocol of Popes and Royals.
Le Privilège du blanc
“Privilege of the White” describes the tradition of allowing Catholic queens, princesses, and archduchesses to wear white in the presence of the Pope. You can view a complete list of who’s eligible to wear white, as well as “controversial beneficiaries” here.
Others should wear black gowns, arms covered, hemlines beneath knees, and black tuxedos or suits. However, royals and many women dignitaries have begun wearing different colors than the traditional black. Most notably, Queen Elizabeth wore blue during her last meeting with His Holiness, back in January.
As the Pope is the head of the Catholic Church, and Vatican City, it is appropriate for Catholic royals to genuflect, and, should he extend his hand, to kiss his ring. For non-Catholics, it is a sign of respect to curtsy or bow, however, not mandated.
The Pope, in turn, is often seen bowing politely to royalty, even those of different faiths.
Royals, as well as non-royals, should address him as Your Holiness or Holy Father.(Source.)