#define redPin 9
#define greenPin 10
#define bluePin 11

void setup() {

  pinMode (redPin, OUTPUT);
  pinMode (bluePin, OUTPUT);
  pinMode (greenPin, OUTPUT);
}

void loop() {
    for (int i=0; i <= 255; i++) {
      analogWrite(redPin, i);
      analogWrite(bluePin, 255-i);
      delay(20);
    }

    for (int i=255; i >= 0; i--) {
      analogWrite(redPin, i);
      analogWrite(bluePin, 255-i);
      delay(20);
    }
}