Privilège du Blanc (privilege of the white)
The privilege du blanc is a right that Catholic queens/grand duchesses have when visiting the pope. It means that they are allowed to wear white while in the presence of the pope. Everyone else should (I say should because it it not an obligation, just strongly recommended based on tradition) wear black. The monarchs that currently have this privilege are Queen Sofia of Spain, Queen Paola of the Belgians, Queen Fabiola of the Belgians and Grand Duchess Maria Teresa of Luxembourg.
However, today Princess Charlene wore white in presence of the Pope. We can see that back in 1959, Princess Grace of Monaco wore black because she did not have the privilège du blanc. So, was Princess Charlene granted permission to wear white? Monaco is a Catholic country. Or did she just make a gaffe?
UPDATE: The Vatican released a statement saying the the privilege has been extended to Princess Charlene.