I just finished a quilt on the longarm using a dark green in the bobbin and off-white on top. I was a little worried about it, because I know there are some big name longarm quilters who will only use the same color top and bottom, but in this case the dark thread didn't poke through to the top, and the light thread doesn't show on the back. If you're only seeing the dark thread through the needle holes it should be fine after it is washed, but if the dark is actually being pulled through to the top then you will need to adjust your tension.