Welcome to QB!
You can try unwinding a couple yards of thread and swirling it around a small area of the quilt top. Unwind another color and swirl it around another area, etc. Stand back and see which you like. Usually, the color of thread that is used is personal preference, like so many aspects of quilting. Some people use a color that matches the back. Sometimes a 'neutral' color will blend okay with both light and dark fabrics, like a cream, a taupe, or a gray, even when it's not a color of any of the fabrics used.