RST has some good advice. If I'm doing very dense quilting, I like to use a matching color so that the texture is visible but the heaviness of the thread does not distract from the piecing (unless you are thread painting!). This might mean changing threads several times. If you have a great deal of white, especially if it links together so you could FMQ from one background area to another, I'd probably go with white there. Then depending on your colors, a tone (grayed) color would look good with gray or taupe or a sage green, more saturated colors will be trickier--pull out several colors and puddle some of each on the quilt top, overlapping the colors you want to quilt to see what looks best. I'm always surprised when something that I really didn't think would work, does! In a class with Jamie Wallen this summer, he said that he often used a tone lighter than the colors of the block so the quilting doesn't overwhelm the piecing.