Impressionist color of shadow
Once you begin to closely observe color, one of the first things you will realize is that shadows are not simply "black." True shadows subtly incorporate hints of other colors. Realistic shadows cannot be created by merely grabbing a tube of black paint.
"No shadow is black. It always has a color. Nature knows only colors [...] white and black are not colors." - Pierre-Auguste Renoir
With black banished from their palettes, the Impressionists used the new theory of complementary colors for their shadows, mixing an object's complementary color into its shadow.
"Color owes its brightness to force of contrast rather than to its inherent qualities [...] primary colors look brightest when they are brought into contrast with their complementaries." - Claude Monet
When Monet painted the interior of the Saint-Lazare station, the steam trains and glass roofs created dramatic highlights and shadows. Using a staggering array of browns and grays combined with cerulean blue, chrome yellow, cobalt blue, crimson lake, emerald green, ivory black, lead white, ultramarine, vermilion, and viridian, no shadow was painted without color. Even the deepest of shadows hinted green and purple.