Another vote for a medium light gray. I use small stitches and my threads don't show. I did spend many years trying to match threads to fabric and ultimately decided it didn't matter as much as I once thought.
I do almost all scrap quilts where it is pretty impossible to match threads without going crazy. To help with my vision issues, I am typically threaded with a gray thread and a beige thread, one each top and bottom. When I wind bobbins I do 4-5 of whatever color and switch then when the bobbins run out.