I bought flannel from Joann's that was on sale just for that reason because I was running out of fabric I could use for backs.

I use 108" wide fabrics for my backing. I can usually find a coordinating color with the front and, frankly, I use a lot of gray. My expectation is that the quilt will get a lot use and using a light color on the back doesn't seem practical. However, if the quilt is going to be used as a bedspread, then I use the backing provided by my long-arm quilter. Especially if there is any custom quilting. I feel the end product is better.

I usually use what I have if it works. I do not like a plain background as I do my own FMQ and any little "whoops" shows on plain fabric.