#include <Adafruit_NeoPixel.h>

int PIN = 12;
int N_LEDS = 40;
Adafruit_NeoPixel ring = Adafruit_NeoPixel(N_LEDS, PIN, NEO_GRB + NEO_KHZ800);

void setup() {
  ring.begin();
}

void loop() {
  for (int i = 0; i < N_LEDS; i++) {
    ring.setPixelColor(i, ring.Color(0, 0, 0));
    ring.show();
    delay(50);

    if (i < 10) {
      ring.setPixelColor(i, ring.Color(255, 0, 0)); // red
      ring.show();
      delay(50);
    } else if (i >= 10 && i < 20) {
      ring.setPixelColor(i, ring.Color(0, 255, 0)); // green
      ring.show();
      delay(50);
    } else if (i >= 20 && i < 30) {
      ring.setPixelColor(i, ring.Color(0, 0, 255)); // blue
      ring.show();
      delay(50);
    } else {
      ring.setPixelColor(i, ring.Color(255, 255, 255)); // white
      ring.show();
      delay(50);
    }
  }
}