I believe the following is an updated list of women eligible for this --

The 2 queens of Spain
The 2 queens of Belgium
The 2 grand duchesses of Luxembourg
The princess consort of Monaco
The princess of Savoy

Correct?

Also, it was explained that Catholic princesses during their minority can wear white in the presence of the Pope; but once they reach their majority, they have to wear black.

What this means is that as a girl, Crown Princess Elisabeth of Belgium started off being able to wear white (just like Princess Alexandra of Luxembourg), but had to change to black after becoming a legal adult at the age of 18, but will one day be able to wear white again -- once she succeeds to the throne as queen regnant.

Correct?

I ask this because of a recent visit to the Vatican by the new sovereign couple of Luxembourg: Grand Duchess Stephanie exercised the white privilege for the first time, in meeting the Holy Father in person.