[etabeta]

As I understand, wool could be bleached in the sun to achieve a white color. Other colors were obtained from plants, fruits, minerals, shellfish etc. It’s an ancient art and a fascinating subject. Here is a short article on textiles during the Middle Ages.

http://postej-stew.dk/2019/05/medieval-fabrics-part-2/

[Albion Wilde]

I'm aware of ancient fabric dying and bleaching; but Mary's robe in that painting is the artists' favorite titanium white—mineral-based, not animal- or plant-based, which would be a warmer white.

One of the substances ancients employed to accelerate bleaching was ammonia derived from urine, also in the "warm" color category.

All that said, it's still it's a great painting!