I consider a couple of things:
- what color is my binding going to be? I like the binding and backing fabric to look good together.
- if I have a very light quilt top, I won't choose a dark backing, as it may shadow through (some of that depends on how opaque the batting is)
- what color thread will I be quilting with, and how much do I want the quilting to show on the back? As a longarm quilter, I match my bobbin thread to my top thread. If I'm using white thread, and my backing fabric is black, that thread will REALLY show. If you are a new quilter and aren't really confident of your quilting ability, you may be happier with a thread that blends in on the backing, so it doesn't show all your 'wobbles'. So then I would choose either a lighter backing, or a very busy print, that will help hide the quilting.
