qmk-firmware/quantum/rgb_matrix_animations/jellybean_raindrops_anim.h

#ifndef DISABLE_RGB_MATRIX_JELLYBEAN_RAINDROPS
RGB_MATRIX_EFFECT(JELLYBEAN_RAINDROPS)
#    ifdef RGB_MATRIX_CUSTOM_EFFECT_IMPLS

static void jellybean_raindrops_set_color(int i, effect_params_t* params) {
    if (!HAS_ANY_FLAGS(g_led_config.flags[i], params->flags)) return;
    HSV hsv = {rand() & 0xFF, rand() & 0xFF, rgb_matrix_config.hsv.v};
    RGB rgb = hsv_to_rgb(hsv);
    rgb_matrix_set_color(i, rgb.r, rgb.g, rgb.b);
}

bool JELLYBEAN_RAINDROPS(effect_params_t* params) {
    if (!params->init) {
        // Change one LED every tick, make sure speed is not 0
        if (scale16by8(g_rgb_timer, qadd8(rgb_matrix_config.speed, 16)) % 5 == 0) {
            jellybean_raindrops_set_color(rand() % DRIVER_LED_TOTAL, params);
        }
        return false;
    }

    RGB_MATRIX_USE_LIMITS(led_min, led_max);
    for (int i = led_min; i < led_max; i++) {
        jellybean_raindrops_set_color(i, params);
    }
    return led_max < DRIVER_LED_TOTAL;
}

#    endif  // RGB_MATRIX_CUSTOM_EFFECT_IMPLS
#endif      // DISABLE_RGB_MATRIX_JELLYBEAN_RAINDROPS