Barb in Louisiana

I'd use white. It will show up in the darker areas, and won't make the white fabric look dirty.

I lay out all the colors I want to use on the quilt and see which one appeals to me. Laying them out and seeing them all together brings clarity for me and makes the choice much easier. What often sounds good, when laid on the quilt really doesn't look good at all.

Edited to add: I am a big fan of contrast on a quilt with my threads, even though I am not a very good quilter. I probably would have started with a red too. And sometimes, just a little bit of a color on the quilt, doesn't look that good, but when you do the whole quilt, the color really takes it up a notch.