Even with scrappy quilts, I like to choose a pattern first, and then make an assessment on the colors and values of the scraps that I have.
In my favorite scrappy quilts, there is at least one element of continuity in them. In the beautiful quilt shown (from American Patchwork and Quilting), the same block is repeated throughout, and the light fabrics are all from a similar color grouping of cream. It gives the eye a place to rest.