char led_red = 23;
char led_yellow = 19;
char led_green = 18;

char led_red_state = LOW;
char led_yellow_state = LOW;
char led_green_state = LOW;

long led_red_previous_millis = 0;
// this variable to store the last time
// the RED led has changed state

int red_led_interval = 1000;
// ON for 1000 ms
// OFF for 1000 ms

void setup() {
  // put your setup code here, to run once:
  pinMode(led_red, OUTPUT);
  pinMode(led_yellow, OUTPUT);
  pinMode(led_green, OUTPUT);
}

void loop() {
  long current_millis = millis();

  if (current_millis - led_red_previous_millis >= red_led_interval)
  {
    led_red_previous_millis = current_millis;

    // now reverse the led's state
    if (led_red_state == LOW)
      led_red_state = HIGH;
    else 
      led_red_state = LOW;

    // control the led accordingly
    digitalWrite(led_red, led_red_state);

  }
}