#define R 6
#define G 5
#define B 4

void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(R, OUTPUT);
  pinMode(G, OUTPUT);
  pinMode(B, OUTPUT);

Serial.begin(115200);
}

// the loop function runs over and over again forever
void loop() {
if (Serial.available()) {
char readk = Serial.read();

    Serial.println(readk);

  if (readk == 'r')
  {
    digitalWrite(R, HIGH);  // turn the LED on (HIGH is the voltage level)
  }  

  if (readk == 'g')
  {
    digitalWrite(G, HIGH);  // turn the LED on (HIGH is the voltage level)
  }  
  if (readk == 'b')
  {
    digitalWrite(B, HIGH);  // turn the LED on (HIGH is the voltage level)
  }            
}
}