I am wondering what's mostly used for backround fabric on quilts.

I like Moda Marbles, Robert Kaufman Fusions ( all of them , so many great ones), And tone on tone muslin type. I like fossil ferns for when I am stuck for a good one to mix with batik.

i like tone on tones

I usually use a tone on tone rather than a solid color & prefer natural color vs whites.

I often use black as a background in my piecing and applique. I am currently using a white-on-white background for an applique quilt from one of Patricia Campbell's jacobean books. I'm using only hand-dyes for the colors. So far, I'm liking it though I must say I rarely use a white background.