Grisaille is a painting entirely painted in shades of greys. It served as an underpainting for an oil painting. The colours are then glazed over it. Imagine you are drawing a three-dimensional cube with its three sides. In a Grisaille you would paint one side in a very darkish grey, which reflects the shaded area, and one side with a very light grey. The third side will be a middle grey between the shaded and the highlighted side. Now, you take any colour of the rainbow and paint over it. The result is a monochromatic cube with its particular shades of hue.