It didn’t take me and Robert long to find an RGB LED pushbutton. I composed a short part number using the NKK data sheet and found a KP0215ASBKG03RGB-2SJB. I made a simple perf board shield with the proper resistors for my Arduino Mega 1280 and re-learned Arduino to light it up.
When I wanted to smoothly fade between all the available RGB colors, I couldn’t find a good solution. So I made my own using Gaussian curves. Here is a picture and link to the online graphic calculator desmos that was very helpful visualizing the LED levels.
There is more: