How can I scroll through all colors of an RGB LED using an Arduino microcontroller and a single Potentiometer?
Mapping a three dimensional space to a single dimension is inherently difficult!

I'd use an HSV model and use the pot to vary H for fixed S and V. I'd Google for an algorithm to translate HSV to RGB and implement that in the Arduino.

I'm assuming you know how to connect pots to ADC inputs and read them. Also that you know how to connect RGB LEDs and drive them.