Thread color and weight are usually discussed at length in books about quilting the quilt (as opposed to books about piecing the quilt). It all depends on whether you want to emphasize the quilting or not. To audition a thread, you can spill a bunch of it onto the quilt and see if you like the way it looks. I often use different threads on top and bottom, though the tension has to be just right for them not to pull through occasionally to the other side. Just go with what you like, because there are no right and wrong answers.