I love to quilt with variegated threads. For the back, it depends. I've matched the color to the fabric. I've used the same as the front, or used off white. If you're new to the quilting part, you might feel more comfortable blending, rather than contrasting. I've quilted with black, but black on black is very difficult to see where you are quilting. I liked the finished look, but did a bit of cursing along the way. :)