Light, Shadow and Colours.

The artist who seeks glory from the ignorant masses- who only want beautiful colours- and avoids the beauty of shadows which show the marvelousness of relief in a flat object, has no understanding of the real glory of art as it is perceived by the most noble minds.
What is beautiful is not always good. I say this to those painters who love the beauty of colour so much that they intentionally paint only weak, barely perceptible shadows, insisting that this is the right way to paint.
Shadow is a more powerful force than light for it can entirely deprive objects of light, whereas light can never chase away all the shadows of object because these objects are solids.

Treaties on Painting - Leonardo da Vinci