The confusion depends upon the vintage of PA28. An old 180 which I once flew had a vernier type throttle plus similar 'plunger' knobs for mixture and carb heat. No doubt one day the mixture knob had been bright red, but by the day in question it was similar in all appearance to the carb heat, except that it had a slight reddish hue. On final, a student converting to the aircraft suddenly pulled the mixture control fully out on me with the predictable result. Fortunately the engine recovered instantly when the knob was pushed back in!
Who is it that thinks it's a good idea to prat about with anything else other than control column, throttle and trimmer once the final turn is complete and the flaps are fully down? Check the carb heat downwind in a PA28 - then leave the damn thing alone. Always stay within gliding range of a suitable landing surface whenever you can!