When I took my very first quilting class, the teacher told us that the back fabric of a quilt should be no lighter than the lightest fabric on the quilt's top. AND that the color of the quilting thread should match the back. She said this way you will be hiding any quilt stitching mistakes or skips.
I forgot this lesson and once when taking a class, I used a coral thread to emphasize the quilt stitches on the top. They look great, but the back, which was muslin . . . . well that is another story. Needless to say, I have followed the rule since.
Suzanne