Princess Charlene has the "privilege to wear white" when meeting Pope Francis, unlike Queen Camilla.
The Queen, 77, met His Holiness alongside King Charles in a private meeting at the Vatican on Wednesday, amid Their Majesties' State Visit to Italy.
Queen Camilla could be seen wearing a simple black dress, a three-strand pearl necklace and her Van Cleef & Arpels bracelet.
Earlier in the day on Wednesday, Camilla had re-worn her white Anna Valentine wedding dress from 2005 to mark her 20th anniversary.
However, Her Majesty would have been forced to change into a black number to meet Pope Francis due to a historic rule - "the privilege of the white".
Called "le privil ge du blanc" in French or "il privilegio del biacno" in Italian, the special tradition is extended solely to designated Catholic Queen’s and Princesses.
Princess Charlene of Monaco, Queen Letizia of Spain, Queen Mathilde of Belgium, the Grand Duchess of Luxembourg, and the Princesses of the House of Savoy are permitted to wear white when meeting the Pope, who is head of the Roman Catholic Church.
As Queen Camilla is a Protestant and married to the Supreme Head of the Church of England (King Charles), this rule does not apply to her.
The "privilege of the white" is usually reserved for important events at the Vatican — such as private audiences, canonisations, beatifications and special masses.
Princess Charlene wore white during two previous meetings at the Vatican, with Pope Benedict XVI in 2013 and Pope Francis in 2016.
Despite being allowed to wear white, Charlene did opt to wear black during her most recent meeting with Pope Francis in 2022.
Prince Albert's parents, Prince Rainier III and Grace Kelly were received by Pope Pius XII in 1957, Pope John XXIII in 1959 and Pope Paul VI in 1974.
In 2005, Prince Albert's first public appearance as sovereign outside Monaco was his attendance at the funeral of Pope John Paul II.
After meeting the Pope on Wednesday, the British Royal Family wrote on social media: "Che occasione speciale! A special moment for Their Majesties as they privately met His Holiness Pope Francis at the Vatican yesterday.
"The King and Queen were deeply touched by The Pope’s kind remarks about their 20th wedding anniversary and honoured to be able to share their best wishes to him in person."