Your bar is yellow ... that means, if you found a better trainer, you could get more lessons. So far I understand. But: Is there only a yellow bar if any matching trainer exists in game? I doubt that this reverse check is at all implemented in game.
People with maxed stats (level 200, where available) also still have them yellow, as if they could train their stats beyond 200. No matter if a maximum would ever be set greater than 200. So I believe: A yellow bar appearing means that you finished your practice lessons of the last level. Not that a trainer for the next level is indeed implemented already.