plein air landscape painting
Painting From Life vs. From Photos

Underpainting gets its name because it is painting that is intended to be painted over in a system of working in layers. There is a popular misconception that underpainting should be monochrome, perhaps in gray-scales. In fact, a multi-color underpainting is much more useful. The colors of the underpainting can be optically mingled with the subsequent overpainting, without the danger of the colors physically blending and becoming muddy. If underpainting is done properly, it facilitates overpainting. If it seems that one has to fight to obscure the underpainting, it is a sign that it was not done properly.

Here is an example of an underpainting made in acrylic, by Hanneke van Oosterhout. It is fairly monochrome, but this is because of the muted colors of the objects depicted.