Only seven women are currently permitted by custom to wear white when visiting the Pope (the rest should wear black). This is the so-called “privilège du blanc”, which is granted to some Catholic queens or consorts of kings for historical reasons. – WTF Fun Facts
Source: https://aleteia.org/2017/06/01/only-7-women-in-the-world-can-dress-in-white-in-front-of-the-pope/