I want dark green thread on the top of my quilt, and white on the back. Why can’t you do this?
Actually, while this can be done, I usually advise against using two different thread colors for longarm quilting as you might not be very happy with the outcome. With a longarm quilting machine, we are moving the machine around in all directions over the quilt sandwich at a high rate of speed. As we change direction, the thread tension often changes slightly as well. Ideally, the portion of a stitch where the top and bobbin thread meet will lock exactly in the center layer of the batting, but this is not always the case. Then you end up with what we longarmers call ‘pokies’ – little dots of the top thread showing on the backing, or bobbin thread showing on the top of the quilt sandwich. When you use two thread colors that differ widely, these ‘pokies’ are very pronounced.