int redpin = 11; //select the pin for the red LED
int bluepin = 10; // select the pin for the  blue LED
int greenpin = 9; // select the pin for the green LED
char caracter;

void setup() {
  Serial.begin(9600);
  pinMode(redpin, OUTPUT);
  pinMode(bluepin, OUTPUT);
  pinMode(greenpin, OUTPUT);

}

void loop() {
  if (Serial.available()) {
    caracter = Serial.read();

    if (caracter == 'a')
    { digitalWrite(redpin, HIGH);
      digitalWrite(greenpin, LOW);
      digitalWrite(bluepin, LOW);
    }
    if (caracter == 'b')
    { digitalWrite(redpin, LOW);
      digitalWrite(greenpin, HIGH);
      digitalWrite(bluepin, LOW);
    }

  }
}