I like to use variegated thread, but I also have lately started liking gray when the colors are widely different in a quilt. I took a quilt that had a variety of very different colors to a local longarmer and she suggested it. I was very happy with the results. If you don't want the quilting itself to be really noticeable, a gray might be the best choice. Do a couple of test sandwiches with different colors and see what you like.