const int greenLED = 2;
const int yellowLED = 3;
const int redLED = 4;

char lastCommand = ' '; 

void setup() {
  Serial.begin(9600);

  pinMode(greenLED, OUTPUT);
  pinMode(yellowLED, OUTPUT);
  pinMode(redLED, OUTPUT);

    // This is the menu of choices
  Serial.println("Choose one of these letters to turn on the light:");
  Serial.println("R - RED");
  Serial.println("Y - YELLOW");
  Serial.println("G - GREEN");
}

void loop() {
  if (Serial.available() > 0) {
    char command = Serial.read();
    
    if (command == 'g' || command == 'G') {
      digitalWrite(greenLED, HIGH);
      digitalWrite(yellowLED, LOW);
      digitalWrite(redLED, LOW);
       Serial.println("- You have turned on GREEN");
      lastCommand = 'g';
    } else if (command == 'y' || command == 'Y') {
      digitalWrite(greenLED, LOW);
      digitalWrite(yellowLED, HIGH);
      digitalWrite(redLED, LOW);
      Serial.println("- You have turned on YELLOW");
      lastCommand = 'y';
    } else if (command == 'r' || command == 'R') {
      digitalWrite(greenLED, LOW);
      digitalWrite(yellowLED, LOW);
      digitalWrite(redLED, HIGH);
      Serial.println("- You have turned on RED");
      lastCommand = 'r';
    } else if (command != ' ' && command != '\n') {  
      // This is the error message
      Serial.println("Sorry, you have written a wrong letter. Choose one of these (R - RED, Y - YELLOW, G - GREEN)");
    }
  }
}