Thanks. Probably a stupid question on my part, but what should it do? I've got three boxes on three different TV's, but I can't see any difference.
I was expecting a change like from limited RGB to full RGB , but colors and black levels stay the same.
That’s exactly what you shoud be seeing. You would immediately see if you have a color range problem.
RGB mode is not an improvement. It is needed only if your tv requires it.