const int redPin = 11;
const int greenPin = 10;

int val;

void setup() {
   pinMode(redPin, OUTPUT);
   pinMode(greenPin, OUTPUT);
}

void loop() {
   for (val = 255; val> 0; val--) {
      // dull red
      analogWrite(redPin, val);
      // bright green
      analogWrite(greenPin, 255 - val);
      delay(15);
   }
   for (val = 0; val <255; val ++) {
      // bright red
      analogWrite(redPin, val);
      // dull green
      analogWrite(greenPin, 255 - val);
      delay(15);
   }
}