DioderFirmware is a project mainly written in C, it's free.
Firmware for my USB controlled DIODER
This is a slightly modified version of LUFAs GenericHID demo to receive data over HID and set PWM output accordingly to whats received.
It takes commands like "AFF7f00", "Rfe", "gFf", "Be0" to Set all, red, green or blue LED output.