qmk-firmware/users/stanrc85/layer_rgb.c

#include "stanrc85.h"

void matrix_init_user(void) {
  rgblight_setrgb(0xFF,  0x00, 0x00);
};

uint32_t layer_state_set_user(uint32_t state) {
  switch (biton32(state)) {
    case 0:
        rgblight_setrgb (0xFF,  0x00, 0x00);
        break;
    case 1:
        rgblight_setrgb (0x00,  0xFF, 0x00);
        break;
    case 2:
        rgblight_setrgb (0x00,  0x00, 0xFF);
        break;
    case 3:
        rgblight_setrgb (0xFF,  0xFF, 0xFF);
        break;
    default: //  for any other layers, or the default layer
        rgblight_setrgb (0xFF,  0x00, 0x00);
        break;
    }
  return state;
}