Gotta love Arduino!
Made a quick demo to show PWM intensity control of Red, Green, and Blue LEDs in a rotary encoder using the rotary encoder’s push button for channel selection and a 7 segment display to show the channel.

The rotary encoder in the pictures below contains three LEDS (Red, Green, & Blue).  They have a common cathode and so I tied their anodes each through a resistor to a separate PWM output of an Arduino Uno. This allowed me to adjust intensity of each independently with a value from 0-255. I then read the encoder output and push button state of the encoder (did I mention it has a push-button switch in it too? Yuup!) using a few more Arduino inputs. I used the push button to select which RGB channel the user was modifying and the encoder output to adjust the selected value. I then used a very inexpensive 7 segment display from Sparkfun with a built-in controller that accepts serial comms for control.

Using all built-in or easily available libraries from Arduino, I was able to have this entire demo functional in about an hour. After I had it functional, I easily spent hours just clicking the push button, selecting RGB channels and changing the mixture of Red, Green, & Blue. This was a simple demonstration meant to explore very minimal user interface options for input and control.

Total time spent making = 1 hour.
Total time spent mixing colors after the fact = too many!