So most Arduino-alikes have USB support these days, and often appear to the host computer as a serial port. The absolute easiest thing is to take advantage of

So most Arduino-alikes have USB support these days, and often appear to the host computer as a serial port. The absolute easiest thing is to take advantage of this; on the microcontroller end read the color over the serial port and set the LED appropriately. On the host computer end, write a tiny program that opens the right serial port and sends the desired bytes. Super straightforward. From there, being a USB HID device will be a lot easier on your host machine code. You have to speak a predefined protocol, but the OS can use its own drivers to take advantage of the functionality you offer. For example, your microcontroller code can be a keyboard or a joystick. (It's still bidirectional, so you should be able to configure the color of the on-board LED. I don't know whether or not there's a standard there so you can just use a pre-written host app across platforms.) Finally, if you want to pick apart an actual product that does this, I just found: https://www.blinkstick.com/ https://www.blinkstick.com/