I would think the back is just something to hold the quilt together. Sometimes when I run out of material, I just grab some Fat Quarters in complimentary colors and piece it. Other times I just grab a piece of material that somehow matches one of the colors in the quilt and just use it.
Are you planning to FMQ, Hand quilt, or tie the quilt to finish it off? If you are quilting, the back will show off your quilting skill and work (think of it like a wholecloth quilt). Sometimes it is better to make the back dark in order to not have the stitches show. I am doing one right now that has a light tan back because I couldn't find any chocolate colored material to fit. I have flipped the quilt over so I can see what I am quilting on the back since it is a solid color. The front is very busy with prints so I figured it would be easier to control my quilting from the back than the front.
Just do it! It is more important to finish it than to obsess over what is on the back.