That's a puzzle. A basic throttle has three wires. (1) Power, which is around 4.3V. (2) Ground. (3) signal which usually varied between 1.0 volt and 3.5 volt like you noted. Your unit has a speed setting, which apparently works.
How do you pick the three speeds? Is it a push button that sequences 1-3? Is there a start button there too? Did the second throttle come with a wiring diagram with wire info?
Most sane designers would use red for the 4.3 volts though. Just traditional.