There is no reason you can't "frame" your blocks with another color and still use your white sashing fabric. Just think of it as another element of the block and don't think of it as "sashing". I would do what the others stated and frame your blocks with another color and then cut them down to all be the same size. This solves two problems, problem 1 of getting your blocks and panel pieces all the same size and problem 2 of not having enough white. Surely there is another color you can bring into play with the theme of butterflies!