It's Letizia's first privilège du blanc appearance, aww. Vatican protocol traditionally requests black or dark outfits from most ladies meeting with the Pope, but privilège du blanc allows select few Catholic royal ladies to wear white. Spanish queens are among those with the privilege, so the new Queen Letizia got in on the act in her white tailored skirt suit with subtle white embroidery on the cuffs and bottom of the jacket. She did not wear a head covering for the couple's audience with Pope Francis (and neither did Queen Sofia when she and King Juan Carlos visited back in April).
You never quite know what you're going to get, dress code-wise, when royals visit the Vatican. The old protocols that included gala dress, veils, and even tiaras in some cases, is fading especially fast under the current pope. This visit, sartorially, was a huge change from Felipe and Letizia's first visit as a married couple, ten years ago: