Since you said you are still working on your FMQ skills, I would definitely go with a matching color front & back -- which means white, in a 50wt.
The only alternative I'd consider, though I seriously dislike monofilament thread, would be monofilament on the front & then adjusting your tension so that it's pulled slightly to the back (generally hard to see mono pokies). Unless you adjust the tension to force it to be unbalanced, or you have a high loft batting, there is a pretty good chance you'll get some of the thread from the backing poking through on the front. Unfortunately, mono usually doesn't play well in the bobbin.
One other thing I'll mention is that I learned the hard way is even when I did a practice sandwich, to still stop and check the back before I get too much quilting done on the real quilt because sometimes things don't always go as expected on a full sized quilt. Not sure if that's due to the extra drag/fraction from the larger size, or the fact that we sometimes hold/move it differently or what, but I'd just encourage you to stop & check your work before you get too far along -- and keep checking it periodically to make sure it's still looking the way you had hoped.