How do I measure the output voltage of an RGB LED controller?
The controller outputs 12V (likely passes the input straight through), and sinks/grounds the R, G, &amp; B pins through a n-channel mosfet or bjt transistor. There is very little voltage drop present from the controller's active component. All it does is opens/disables the R, G, &amp; B pins (leaves them floating) to turn the leds off, and closes/enables them (connecting the path to ground) to turn the leds on. It uses PWM to adjust brightness and color.

Typical insides:


Google "RGB Controller schematic" for plenty of ideas on how to implement your own. You could even cut the circuit in your controller up to remove the microcontroller while keeping the driver section.