int red = 3, green = 4, blue = 5;

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

void loop()
{
  while(Serial.available() > 0)
  {
    int r = Serial.parseInt();
    int g = Serial.parseInt();
    int b = Serial.parseInt();
    if(Serial.read() == '\n')
    {
      r = 255 - constrain(r, 0, 255);
      g = 255 - constrain(g, 0, 255);
      b = 255 - constrain(b, 0, 255);

      analogWrite(red, r);
      analogWrite(green, g);
      analogWrite(blue, b);

      Serial.println(r, HEX);
      Serial.println(g, HEX);
      Serial.println(b, HEX);
    }
  }
}