int ledR = 3;
int ledG = 5;
int ledB = 6;

void setup() {
  Serial.begin(115200);
}

void loop() {

  String command = Serial.readStringUntil('\n');

  if (command.length() > 0) {
      int led; 
    if (command[0] == 'r') { 
      led = ledR;
    }
    if (command[0] == 'g') {
      led = ledG;
    }
    if (command[0] == 'b') {
      led = ledB;
    }
    String brightness = command.substring(1);
    analogWrite(led, brightness.toInt());
  }
}