int rot = 9;      // RGB-LED: Farbe rot an PWM-Port 9
int gruen = 10;   // RGB-LED: Farbe grün an PWM-Port 10
int blau = 11;    // RGB-LED: Farbe blau an PWM-Port 11

void setup() {
  Serial.begin(9600);
}

void loop() {
  for(int i = 255; i >= 0; i--) {
    analogWrite(rot,i);
    analogWrite(gruen,255-i);
    delay(20);
  }
  for(int i; i <= 255; i++) {   // ohne "int" bei i --> Fehler "name lookup of 'i'"
    analogWrite(rot,i);
    analogWrite(gruen,255-i);
    delay(20);
  }
}