Backgrounds are an important subject. Since we paint dark colours thinly, the background colour often shows through. This can have a big effect on the finished picture. Here is an animation of a strong red painted background being used as the basis for a simple sky and tree blossom sketch.
Click the video box below to view the animation.
The point about the animation above is that the use of the red background heavily influenced the tonal balance of the finished piece. Seeing the paint against the red directly affected the choices made by the artist during the painting process.
Red was chosen because it is opposite blue (the real colour of the sky) on the colour wheel.
This technique of underpainting in contrasting (or complementary colours was used by a number of the old masters. Duhrer, Titian and Reubens are all believed to have used variations of this technique.