I use either the lightest gray colored thread, or a very light green color. I have found that both of those colors blend well with any color fabric. The light green, I found completely by accident. I was piecing a quilt one day, in that time of day when it is not quite dark enough to turn on the lights. I picked up (what I thought was) my gray thread, and went to town. When I actually turned the lights on, I saw that I had picked up a very light green thread, instead. I thought I would have to undo everything I had done, but to my surprise, I blended just as well as the gray thread.