void setup() {
  Serial.begin(9600);
  Serial.println("Please enter a COLOUR:");
  pinMode(13, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(10, OUTPUT);
}

void loop() {
  if (Serial.available()) {
    String J = Serial.readString(); // Read the integer
    J.trim();
    Serial.print("You entered: ");
    Serial.println(J);
    lightup(J);
  }
}

void lightup(String color) {
  digitalWrite(13, HIGH && color.equals("RED"));
  digitalWrite(12, HIGH && color.equals("GREEN"));
  digitalWrite(11, HIGH && color.equals("BLUE"));
  digitalWrite(10, HIGH && color.equals("YELLOW"));
}

/*
RED
  digitalWrite(13, HIGH && TRUE);
  digitalWrite(12, HIGH && FALSE);
  digitalWrite(11, HIGH && FALSE);
  digitalWrite(10, HIGH && FALSE);

  digitalWrite(13, HIGH);
  digitalWrite(12, LOW);
  digitalWrite(11, LOW);
  digitalWrite(10, LOW);
*/