I was going to use a muslin for the background too but all the muslin I have seen seems a bit too thin and see through. I might have used it if I found some heavier stuff. I saw online somewhere that Moda has a muslin it looked good but can't really tell on the screen. I just did a google search and it was in there somewhere. Anyhoo I decided to just use different light colored batiks instead.
My DM just bought me the last of the lights it was a pale lavender so I think I am good to go.