For me it depends on what the quilt was made for............a gift or utility. If a gift I usually try to buy the backing when I acquire the fabrics for the front or if I've pulled them from my stash then I start to look for the backing right away. Luckily I have some extra backings in my stash and have decided for all 12 quilts which backing I'll use when I get my mojo back. As I've placed myself on a "NO BUYING" fabric since Nov 2018 I've done quilt well. Having no sewing mojo helps too but need to get busy again now that things have slowed down outside in the gardens.
As to your quilt I was think a grey if not too dark would look okay or even a creme if that's what I'm seeing in your quilt.