Originally posted by Tygert
It is a bright green when you have recently moved but if it has almost been ten minutes and you haven't moved, it goes a lighter green to show that you are probably going offline.
Actually that is not quite correct.
It in no way indicates that you are still online or might be about to go offline.
If you make a move and immediately log off, that green dot will still remain for 10 minutes, even if you are long gone.
And after 10 minutes you could still be online but the dot will be gone if you haven't made any more moves.
So if you see that dot, the only thing it means is the person made a move within the past 10 minutes.