I've been reading a book recommended to me by Susan Cook, Vision and Art: The Biology of Seeing by Margaret Livingstone (a neurophysiologist). In it she addresses many characteristics of human vision and how they relate to art in a very palatable way.
Recently, one section really caught my attention. In it she writes about how we see 3D vs 2D and how important luminance, or value, is to create the illusion of depth. I heard this plenty in school (judge values first, then color). I guess it's just on my mind a lot recently so this was a really great reminder.
Two examples really struck me. The Woman with a Hat (la Femme au Chapeau)  shown below in color and then in grayscale. The colors look wild and yet there is some depth to the figure, some planes of the face defined. Viewed in grayscale it becomes more apparent that it's the relative values of the colors that give the piece dimension.