THREE ARTIST WHO USE GOLD

Three very different artist who use gold as a ground. What does it do in each work? How do the later works reference the earlier ones?

From top to bottom: Leonardo Davinci's Madonna and Child, 16th century-ish. Gustave Klimt,Portrait of Adele Bloch-Bauer I 20th century. And last but not least, Tenyouya Hasashi, 21st century,

Japan.