I used RED flannel on my very first quilt for batting. I knew NOTHING about quilting, was pregnant and made hand-embroidered quilt blocks for my child (40 years ago). After I set them together, I had a bunch of red flannel, so I washed it and used it. Worked fine but after 40 years the red is showing up at the seams.
But my daughter still loves it so it worked just fine.
Kathy