Ultramarine

Natural ultramarine was ground from lapis lazuli, a semi-precious stone found in commercial quantity in only a few locations in the world (historically, primarily Afghanistan), then shipped enormous distances before it ever reached a European painter's workshop — the cost reflected genuine material scarcity and transport distance, not an arbitrary price.

In practice: This is exactly why Renaissance painters reserved natural ultramarine for their most important subjects — a real economic constraint, not an artistic rule about which subjects 'deserved' blue.

Because natural ultramarine was so costly, a patron commissioning a painting could signal genuine devotion (and wealth) by paying for it to be used on the most important figure in the scene — in much of Renaissance European religious painting, that was very often the Virgin Mary, which is why her robes are so frequently blue.

In practice: This convention says more about historical pigment economics than about any inherent meaning of the colour blue itself — a useful reminder that a 'traditional' colour choice often has a material explanation, not just a symbolic one.