You have to have special permission to wear white in front of the pope. Princess Charlene of Monaco is one of the few who have that honor.
Of course, wearing black (or white) is not required for women who meet with the pope. As head of the Church of England, Queen Elizabeth II has previously worn black and a mantilla when meeting with Pope John Paul II and Pope John XXIII, but she hasn’t followed the protocol in more recent visits
https://people.com/royals/princess-charlene-of-monaco-and-the-privilege-of-the-white/