qmk-firmware/users/drashna/readme/rgb.md

Layer Indication Code

At least for RGB Light, the layer_state_set function is used to detect the current highest layer, and change the underglow based on that layer.

This works for both the regular layers, and for the default layers, too.

I use the sethsv variants of the commands, so that different modes can be used, as well.

RGB Matrix uses a custom, per board implementation, at the moment.

RGB Light Startup Animation

On startup, if enabled, the board will cycle through the entire hue wheel, starting and ending on the default layer color.

void keyboard_post_init_rgb(void) {
#if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_STARTUP_ANIMATION)
    if (userspace_config.rgb_layer_change) { rgblight_enable_noeeprom(); }
    if (rgblight_config.enable) {
        layer_state_set_user(layer_state);
        uint16_t old_hue = rgblight_config.hue;
        rgblight_mode_noeeprom(RGBLIGHT_MODE_STATIC_LIGHT);
        for (uint16_t i = 255; i > 0; i--) {
            rgblight_sethsv_noeeprom( ( i + old_hue) % 255, 255, 255);
            matrix_scan();
            wait_ms(10);
        }
    }
#endif
    layer_state_set_user(layer_state);
}

This could probably benefit from some cleanup and better handling.

RGB Light Twinkling

This enables random twinkling of the LEDs when typing.

RGB Light Mod Indicators

Allows feedback of which mods (oneshot or otherwise) are enabled.