#include <FastLED.h>

#define NUM_LEDS 10
#define DATA_PIN 6

CRGB leds[NUM_LEDS];

void setup() {
  FastLED.addLeds<WS2812B, DATA_PIN, GRB>(leds, NUM_LEDS);
  FastLED.clear();
  for (int i = 0; i < NUM_LEDS; i++) {
    leds[i] = CRGB::Grey; // Nicer than black
    FastLED.show();
  }
}

void loop() {
  for (int i = 0; i < NUM_LEDS; i++) {
    leds[(NUM_LEDS - 1 - i) % NUM_LEDS] = CRGB::Green;
    leds[(NUM_LEDS     - i) % NUM_LEDS] = CRGB::Grey;
    FastLED.show();
    delay(1000);
  }
  for (int i = 0; i < NUM_LEDS; i++) {
    leds[(               i) % NUM_LEDS] = CRGB::Green;
    leds[(NUM_LEDS - 1 + i) % NUM_LEDS] = CRGB::Grey;
    FastLED.show();
    delay(1000);
  }
}