Muslin is traditional, but a nice '30's repro print with a small design would look nice, too. No reason you have to make it exactly like our great grandmothers did. Why not have something that is beautiful in both sides? Of course, today's muslins are also quite striking and come in many different colors.
But my great grandma used that grayish blue-green muslin for the backs of her feedback quilts. I bought some at a repro LQS recently and my mom & grandma immediately recognized it as the color great grandma had on the back of all her quilts (and as sashing on several fronts). I'll try to post a pic of it when I get home.
Edit: never mind. It's the darker green on the far right of the upper left block in your pic.
Last edited by Bree123; 11-09-2015 at 02:25 PM.