// RGBLED - cycle RGB

byte led[] = { 2, 3, 4 };
unsigned long timer, timeout = 5;

void setup() {
  for (byte i = 0; i < 3; i++)
    pinMode(led[i], OUTPUT);
}

void loop() {
  if (millis() - timer > timeout) {
    timer = millis();
    if (i++ > 3)
      i = 0;
    for (byte j = 0; j < 3; j++) {
      digitalWrite(led[j], i == j); // on new color
    }
  }
}