If you want to use white in the bobbin and a darker color on top -- do so. Sew on a sample first to make sure your tensions are such that the top thread isn't going thru to the back more than you want. Is there a reason that you don't want to use white on the top, too? Start with a normal (or mid #) tension and go from there. Hope this helps. Here's a hint: if the back of fabric looks looping - tighten upper thread tension. (But make sure you have properly threaded your machine -- redo it at least once before messing with tensions.)