Grey works with almost any quilt color. It somehow has the ability to diffuse itself into the quilt, not matter what color you are quilting over. The only exception I would make to that is if your quilt is warm pallet based instead of a cool pallet, like your pictured quilt. In that case, where the tones are earth tones in warm, autumn colors, your go to color would be a medium beige or light tan. I think you will be really pleased with this quilt if you stick to the grey thread. This rule of thumb is also true when piecing a quilt - grey (or beige) works almost every time.
Marcia in Crowley