It looks like you have received a great deal of good information about tension which is very valuable. The choice of color on top and bottom can be very different or the same. I love to use a variegated thread on the bottom if the backing is a colorful print. IF it is more a solid I like to use the same threads on top and bottom. If you are doing a feather pattern when quilting a more challenging design you may want the bottom color matching the backing so that any challenges you face when quilting will not show through to the bottom. Go for it switch up the colors. Practice and see what you like there is really no limits. It all has to do with preferences.