main.cpp

#include <Arduino.h>
#include <FastLED.h>

#define LED_PIN       21
#define NUM_LEDS      1
static CRGB leds[NUM_LEDS];


void setLed(CRGB color) {
  // change RGB to GRB
  uint8_t t = color.r;
  color.r = color.g;
  color.g = t;
  leds[0] = color;
  FastLED.show();
}


void setup() {
  FastLED.addLeds<WS2811, LED_PIN, RGB>(leds, NUM_LEDS);
  FastLED.setBrightness(255 * 15 / 100);
}


void loop() {
  setLed(CRGB::Red);
  delay(1000);
  setLed(CRGB::Green);
  delay(1000);
  setLed(CRGB::Blue);
  delay(1000);
  setLed(CRGB::Black);
  delay(1000);
}

platform.ini

[env:esp32-s3-devkitc-1]
platform = espressif32
board = esp32-s3-devkitc-1
framework = arduino
build_flags = 
	-DARDUINO_USB_MODE=1
	-DARDUINO_USB_CDC_ON_BOOT=1
monitor_speed = 115200
lib_deps = fastled/FastLED@^3.5.0