Kona cotton would be just fine for a quilt back. Just be aware that your quilting will show up more on a solid color than on a print. That might be good, might be bad, depends on your quilting. I always use prints to camouflage any mistakes. I think prints will also hold up better as the quilt gets used. Any dirt and stains seem to show up a lot more on solids. The design of your quilt might also suggest using either a solid (e.g. for traditional Amish designs) or a print.