Painters of experience usually paint an underpainting before beginning a new work. This is called ground. There are several advantages to adding a ground, and different methods in applying depending on several factors.
The first consideration for adding a ground could be color. Usually, the color choice depends on what is going to be painted. In other words, you probably wouldn’t want a burnt umber ground under a brightly lit landscape. The ground would be too dark. Many times, while painting, the ground color will show through in places. This happens intentionally allowing the ground color work for you. I knew an artist who used gold leaf as part of her ground. The small areas that allowed the gold leaf to shine through were stunning. Application of gold leaf as a ground, however, is very time consuming and the material is very fragile. But the end result is very strikingly beautiful.
The color of your ground is usually a middle tone of the other colors of your painting. If you put this color on a value scale, it would hit about in the middle values. While the artist is painting, they are fully aware of this value and will add lighter or darker tones around the ground to achieve the effects they are seeking. Using the ground in this way is smart letting that color work for the painting.
The old masters like Rembrandt probably used a very dark ground to achieve the strong contrasts of dark and light in the work. This technique was pioneered by Titian in the High Renaissance. The colors of the underpainting can be optically mingled with the subsequent overpainting, without the danger of the colors physically blending and becoming muddy.
There are a couple of different ways to apply ground. Usually, the paint is thinned with some form of oil if using oils. Thinly paint on the ground color then wipe off to smooth with a cloth that will not shed fibers. This is very important as your paint will pick those fibers up when you begin work. Or, if you choose you can merely brush out the color to make it consistent. Some occasions you might prefer those variations in the ground color, again depending on the effect of your motif.
If you are underpainting in acrylic with the intention of using oils to finish, make sure you wipe on a thin coat of linseed oil between the paints to bond the oils to the acrylic. I hope these have been a few useful tips for working with ground color.