Your quilt is very beautiful. It appears that you have a multitude of suggested options. I think I would choose one color and use it in several shades for the quilting. Your quilting design could be intricate-or not so much. I would use all black on the back only if you have no tension problems so you wouldn't have black on the front. If you have color on the back you have the opportunity for a secondary design opportunity. I am not a big fan of variegated threads when there is such variety of color in the patches.