This is the first little Arduino program I wrote (outside of basic tutorials). It turns on a blue, red, or green LED based on a key stroke. Neat!

blue_red_green.ino

char input = 0;

int blue = 9;
int red = 10;
int green = 11;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  delay(100);

  pinMode(blue, OUTPUT);
  pinMode(red, OUTPUT);
  pinMode(green, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly: 
  
  if(Serial.available() > 0) {
   input = Serial.read();
   handleInput(input); 
  }
}

void handleInput(char i) {
  if(i == 'r') {
		lightOn(red);
	} else if (i == 'b') {
		lightOn(blue);
	} else if (i == 'g') {
		lightOn(green);
	} else if (i == 'o') {
		allOn();
	} else if (i == 'x') {
		allOff();
	}
}

void allOn() {
	digitalWrite(blue, HIGH);
	digitalWrite(red, HIGH);
	digitalWrite(green, HIGH);
	Serial.println("All On!");
}

void allOff() {
	digitalWrite(blue, LOW);
	digitalWrite(red, LOW);
	digitalWrite(green, LOW);
	Serial.println("Lights Out!");
}

void lightOn(int color) {
	digitalWrite(color, HIGH);
	Serial.print(color);
	Serial.println(" on.");
}