Softwarewise not much to expain.
I used platformio - i think that's the best option today to do arduino boards development.
I flashed latest optiboot to go for almost zero boot time. With stock bootloader it took ~2 seconds to white light to switch on which is really annoying.
To control the state of light before arduino code is running i had to shunt enable pins to ground and led pint to high level with 10k resistors. Otherwise it would blink with some random colors before white light is on
Main code is a state machine and more or less self explanatory.