#define red 13
#define green 12
#define blue 11

void setup() {
  Serial.begin(9600);
  pinMode(red, OUTPUT);
  pinMode(green, OUTPUT);
  pinMode(blue, OUTPUT);
}

void loop() {

  if(Serial.available()) {
  char rx = Serial.read();

    switch (rx) {
      case 'r':
        digitalWrite(red, HIGH);
        digitalWrite(green, LOW);
        digitalWrite(blue, LOW);
        break;

      case 'g':
        digitalWrite(red, LOW);
        digitalWrite(green, HIGH);
        digitalWrite(blue, LOW);
        break;

      case 'b':
        digitalWrite(red, LOW);
        digitalWrite(green, LOW);
        digitalWrite(blue, HIGH);
        break;
    }
  }
}