In rule discovery games, it really helps to be able to return to previous levels in order to experiment with the rules that are still unclear. Unfortunately there's no level select here. I got to the 4 yellow doors but I can't find a way ahead, I'd say the rule required at this point is quite underclued.
I agree that some mechanics would benefit from more introductory levels, especially the blue knob. There is a level select though, if you right click to return to the main menu and click the levels select button, you can go to any level. Completed levels will display as green doors and incomplete levels will display as red doors. Your progress should save between sessions.