I'd probably go with soft grey. Moda has one called Zen grey that I like and have used.

I like your block quilt idea. Your fabrics do look great side by side.

Have you thought about asking your GD which pattern she likes?

Beautiful fabrics & I love the simple block quilt. Also I would use the med to dark gray for sashing if you choose a quilt with sashing.

My chosen white these days is vanilla. It's white, but with a slight beige/tan/ecru/bone feeling. It's a crisp contrast, but it doesn't shriek "w-h-i-t-e" next to colors. You have some really beautiful fabrics in your collection. I'll be you will be delighted with the finished quilt. Your granddaughter will be ecstatic.

My vote is the top print in row 2.

If you really want to use a sashing I would go with black, but honestly I think it I would do it without the sashing. Keeps the quilt simple and no one color would be showcased..I'm sure she will be thrilled with whatever you decide to do. Please post when you are finished, would love to see..

I like to sew my left over fabric into strips and use it in my binding sometimes.

I would use a medium grey like the one at the bottom center of your colors. Why, It will let the wonderful purples colors shine while not overpowering the design.

Love all the colors you are using. It is going to be great whatever you choose.
peace

The light pinkish in the upper left hand corner!

Those are my two choices also!