Often the background fabric will be a light solid or tone-on-tone fabric. These types of fabrics allow the colorful pieced design to stand out. Most patterns that you purchase assume that you will use the lightest fabric for the background.
Darker fabric backgrounds aren’t as popular but can also accentuate the pieced pattern. The more contrast between the background and the pieced design, the more the pattern will shine.
In the photo below, a dark green fabric (so dark it is almost black) was used for the background.