It depends on the look I am looking for. I have used one color sometimes if I want a real traditional look. Presently, today as a matter of fact, I am quilting a quilt for a grandson's graduation. I plan to use three different threads. For the major motifs I want to emphasize the fabric pattern I will use a matching thread. For borders I will use contrasting threads to show up the quilting motifs. The quilt has a pieced border and I will do SID to allow the piecing to show up-it took more time to make the border than all the rest of the quilt and I want it to show.