This is a simple Arduino Nano project that color cycles on three pins (3, 5, 6) dedicated to R, G, and B respectively.

I use linear interpretation to go from one color to the next so that you get nice smooth transitions even when very gradual.

The outputs in my case are actually connected to 13005N transistors that drive the common anode legs of a 10W or 50W LED inside a 1955 Pontiac hood ornament.

Dave Plummer