I'm usually too lazy to backtrack too far on a project, but have once or twice changed my mind and redone one fabric that wasn't used a whole lot.
Maybe it could be rescued with some special quilting with variegated thread or with the addition of some appliques. Or another idea might be to find a different fabric to use as cornerstones or a border that would somehow make that background play differently. I love Watson's idea of fabric paint, but would have to wonder how that could be controlled so that it wouldn't bleed into adjacent pieces. Crayolas can be used and provide a pretty stable color, but it would probably be more work than changing the fabric because you have to press the wax out with an iron and paper towels. A really wild approach might be to dye the entire piece in a pastel shade that would not only change the background but put a new "spin" on all the other colors. You'd have to be brave and devil-may-care to try that! I'm thinking of something along the line of tea dying. Probably there are good tutorials online for that process.