If I were going to sash it, I would try out a yellow sashing around two blocks, so you would be framing a whole mountain. It will either look really good or really bad, and I'm not sure which. The right colors and combinations of border would be an easier option, though.
I have a kaufman color swatch (for lack of a better term). It has all the solid colors Kona Cotton comes in printed on 2/3 yard white fabric. I bought it at a quilt show, but I would guess it's easy to find online. It's not exactly true to color, but it's close. I've used it several times to find the color I need or someone else needs so that it can be ordered online.