With Hand Quilting, it is your preference what color you use. I usually use a white or ecru thread on everything. It depends on what you want it to look like. Mostly we want to see the effect of the quilting, but not the actual stitches, so we use a thread that blends in. I personally do not like to use black on black because these old eyes cannot see where I have been when the quilt is in the frame. It is the most difficult to work with. If you want to change colors when you are quilting the top to match the color of the fabric on top, use a multicolored fabric on the back to make it less noticeable there. But if you want to see it, use a solid fabrciback. It is your quilt, you can do anything you want. If you are entering it in a contest, check their rules!