A painter should begin every canvas with a wash of black, because all things in nature are dark except where exposed by the light.

This quote means that an artist should always start their painting with an underpainting of black; black is the color of the shadows that paint the natural world. Everything around us is dark in the shadows, and only the areas exposed to light appear bright. Using black as a base for a painting helps to give the painting realism, by creating a contrast between the light and the dark.