As the others have said .... white!
Although, without our seeing the quilt, it's hard to say. There's lots of possibilities without it being totally white. And the mood fo the front could influence what the back should be like.
I'm not partial to "white" by itself, and would probably look for a compatable "nothing print" that works with the front. Or use a combo of the front fabrics for a patched back.
Remember ..... there's no rules .... and the right way, is your way! :)