'When the triptych's wings are closed, the design of the outer panels becomes visible. Rendered in a green–graygrisaille, these panels lack colour, probably because most Netherlandish triptychs were thus painted, but possibly indicating that the painting reflects a time before the creation of the sun and moon, which were formed, according to Christian theology, to "give light to the earth". It was common for the outer panels of Netherlandish altarpieces to be in grisaille, such that their blandness highlighted the splendid colour inside.' Wiki
I never knew this!! SO beautiful!
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