Jeanne brought up an interesting point that I hadn't considered. If you're using an invisible stitch and *don't* want your stitches to show, then you'd probably want to use a neutral or matching thread color, silk or fine thread. If, however, you're doing a blanket stitch and *do* want your stitches to show, then a heavier thread in a darker color (like dark blue on a light blue fabric) or even black would look nice.