I would not use sashing at all. I think the blocks will look lovely together. Use the other fabrics to make narrow white inner border and a wider blue outer border, then bind with blue, white, and yellow scraps. A quilt takes on a totally new look (always for the better) when the blocks are sewn together and quilting is added. I recently quilted a 4-patch comfort quilt that another guild member put together from miscellaneous blue and red squares of donated fabrics. I didn't have too much hope for it when I first saw the top, but you wouldn't believe how pretty it was after it was finished, and I didn't do anything fancy with the quilting. (Sorry, I forgot to take pictures.)