I can see your problem. If you add white, it will mess up the white squares and colour will mess up the coloured squares. I think I would look for a bold fabric that reflects a lot of the colours you've used and do a four inch border of that. Pick a large floral that goes with your colours but isn't in the quilt so it wouldn't blend with a coloured square? Guess I'm not much help, maybe someone else will come up with something brilliant.