const int redLed = 2;
const int yellowLed = 3;
const int duration = 100;

void setup() {
  pinMode(redLed, OUTPUT);
  pinMode(yellowLed, OUTPUT);
}

void loop() {
  fade(redLed, 10, 10);
  delay(500);
  fade(yellowLed, 10, 10);
  delay(500);
}

void fade(int ledPin, int inc, int dec) {
  for (int bright = 0; bright <= 255; bright += inc) {
    analogWrite(ledPin, bright);
    delay(duration);
  }

  for (int bright = 255; bright >= 0; bright -= dec) {
    analogWrite(ledPin, bright);
    delay(duration);
  }

  analogWrite(ledPin, 0);
}