It's uncommon to find a piece of fabric in my stash large enough to back a quilt! Apparently, I've been saving it for?!?!
I need to be sure the fabric is colorfast since the finished quilt is certain to get washed. Of course there are commercial products to stabilize dyes......and I plan to use one. But here's an additional test I also use. I safety pin a small piece of muslin onto the fabric. Wash and dry as usual.....if there's any dye residue on the muslin......I can decide if it needs further washing/treatment, etc.
Generally I do this on every washable piece that enters my house......but by the pristine look of these folded 6 yards, I didn't test it.