RGB LED is used to show layers default bluepill LED is capslock. there's a simple serial protocol for the keyboard to listen for color change commands from the PC. Useful for "do stuff && cat "green">/dev/ttyACM0"
lower case letters set pre-programmed colors Upper case letters change RGB mode Command C takes 3-5 octets of RGB settings. Numbers can be terminated with a comma or period. 3 octets = set all LED, 4th argument specfies specfic LED, 4+5 specify start and stop LEDs.