Sewbiz, you don't need to supply the other fabrics, some do who want a more controlled look to the quilt.
You can specify the one color (white or wow for instance) or multiple colors that can be used, or a specific color not to use (like what you will use for the sashing for instance, or something you just don't like) or any version of the above.
Asking for the one color (white for instance) will limit some pattern choices b/c of only using 2 color patterns, but will give a more uniform quilt. Adding more colors will give more of a scrappy look to the quilt, but allow for wider pattern selection. All up to you and your vision of your quilt.