If you have some fabric left from the quilt top, now would be your chance to add some of that in the backing. If you decide to go with that option, you could put it at one end, and use that as a turn back on the bed, or perhaps you could cut what you have in half, horizontally, and add a piano key strip through the middle. (Piano key strip is made of short lengths of all your fabrics, sewn together a la piano keys. Repeat the fabrics for as long as you need the strip to be).