Originally Posted by ProDigit
... Have you tried changing one of the level values to 'device 0' or '-1'?
If it would work, the system would send a negative voltage to the leds, which only allow current flow to flow in the positive direction, so they should block the light. ...
Not negative voltage, but negative time, because the LEDs are controlled with PWM (Pulse Width Modulation). How do you propose sending them a negative pulse width (where the pulse ends before it begins)? Current understanding of physics would require a moving wormhole to accomplish that.
Because modern white LEDs are really blue LEDs with a white phosphor coating (similar to fluorescent lights), varying the voltage would not work as expected. Even colored LEDs are much more efficient with PWM control than by varying their voltage. Also, LEDs can self-destruct with a reverse voltage applied to them (as little as a few volts in some cases, but 5V reverse breakdown is typical).
EDIT: As I posted about three months ago, value zero does turn the paperwhite screen illumination completely off:
How could reverse-biasing the LEDs make them even darker than applying NO voltage at all?