I find I have to use the same color almost on top and bottom, unless the top is variegated. I use a lot of that. If I'm doing a white area on the top, I switch to a white bobbin. I use Bottom Line for my bobbins on my longarm. Lots of colors. If you use the same kind of thread all the time for your bobbins, switching colors doesn't make your tension change.
Cynthia