Underpainting is the first layer of paint on a panel or canvas, laid in one or two muted tones before any real color goes on.
It works like a map. It fixes where the figures stand and where the light falls. The painter then builds the colors over it in thin coats, and in a finished picture this base ends up completely buried.
Because the shadows are already in place underneath, the colors on top can stay thin. Seen from a few steps away, a thin red over a dark brown base reads as a deeper red, as if the two had been mixed on the palette.
Dutch painters of the seventeenth century had a name for this first stage. They called it the dead coloring, probably because it had almost no color in it yet.
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Since the underpainting is made to disappear, the easiest place to see it is a picture that was never completed. The example most books give is the Adoration of the Magi by Leonardo da Vinci, now in the Uffizi, the great art museum of Florence.
Leonardo left the panel behind in 1482, when he moved to Milan. Much of its surface is brown, and that brown is usually called his underpainting.
Part of the brown, though, is a varnish Leonardo never applied. In later centuries, the people who looked after the panel brushed a tinted coat over it on purpose, so that the unfinished picture would pass for one deliberately painted in a single color.
Leonardo's Adoration under the varnish
That varnish also hid the landscape in the background…
What You'll Discover
How an underpainting differs from a drawing.
Where it sits between gesso and glazes.
What is missing from Michelangelo's Entombment.
Why some old faces look green.
How Rembrandt changed the first layer.