I read somewhere that vintage quilts (pre 1940) were backed with either muslin or white material. Vintage quilts did not have prints or solid colors on the back. Any thoughts?

I think that was often the case but not a rule. Could depend on the intended use of the quilt. I have seen many more utilitarian quilts with feed sack and scrappy backs.

Look forward to reading what others think.

All the old quilts I have are backed with muslin. Although my kin folk used what they had. Times were hard.

My mother-in-law would turn over in her grave if she could see anything on a quilt back except white. All were muslin or feedsacks.

All the quilts my grandmother & great-grandmother made used "flour" sacks for the backing. It feels like a cross between muslin and linen. They used those flour sacks for everything. Dish towels, diapers, toaster covers, napkins, slips, etc. When flour started coming in paper instead of cloth--then they used whatever fabric was cheapest--usually muslin. I think they even called the flour sacks muslin but it doesn't feel anything like the stuff we call muslin now days.

i use bleached or unbleached muslin..tea dyed also, on the backs of quilts..bby and toddler, i piece the backs with the leftover fabrics from the front.

i can't afford sometimes,6 yards, of matching fabrics for the backs..

I have used a decent quality muslin on bed quilts, but Ive been making lap quilts now, so I prefer some color and/or pattern. Both sides of a lap robe show at times, whereas only the top shows on a bed.