void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
  pinMode(3, INPUT_PULLUP);
  pinMode(4, INPUT_PULLUP);
}

void loop() {
  if(digitalRead(2) == LOW){
    Serial.write("green\n");
    delay(500);
  }

  if(digitalRead(3) == LOW){
    Serial.write("red\n");
    delay(500);
  }

  if(digitalRead(4) == LOW){
    Serial.write("blue\n");
    delay(500);
  }
}