I would choose a thread color which will closely match whatever fabric I will be stitching on along the top, or whatever color fabric is in the majority on the top. Then I choose the backing dependent on that thread color. If I want it to blend in and not contrast, I choose a like colored backing with a pattern to it. If you want the stitching to show a lot on the back, choose a backing fabric in a contrasting color with no pattern (solid).