A small bit of code to gran data from Bluetooth via serial and update an LED state.

Peripheral.pde

long previousPrint = 0;        
long intervalPrint = 10000;
boolean red = false; 
boolean blu = false;
boolean gre = false;
boolean all = false;

char val = 'a';

void setup() {
  
  Serial1.begin(115200);
  while (!Serial1);
  
  pinMode(3, OUTPUT); //red 
  pinMode(4, OUTPUT); //blue 
  pinMode(5, OUTPUT); //green
  
}

void loop() {
  
  unsigned long currentMillis = millis();

  val = (char)Serial1.read();

  if (val == 'r') {
    blu = false;
    red = true;
    gre = false;
    all = false;
  } 
  if (val == 'b') {
    blu = true;
    red = false;
    gre = false;
    all = false;
  } 
  if (val == 'g') {
    blu = false;
    red = false;
    gre = true;
    all = false;
  }
  if (val == 'a') {
    blu = false;
    red = false;
    gre = false;
    all = true;
  }
    
  if (gre) {
    digitalWrite(5, HIGH);
    digitalWrite(4, LOW);
    digitalWrite(3, LOW);
  } 
  if (red) {
    digitalWrite(3, HIGH);
    digitalWrite(4, LOW);
    digitalWrite(5, LOW);
  } 
  if (blu) {
    digitalWrite(4, HIGH);
    digitalWrite(5, LOW);
    digitalWrite(3, LOW);
  } 
  if (all) {
    if(currentMillis - previousPrint > intervalPrint) {
        previousPrint = currentMillis;
        analogWrite(3, random(0, 255));
        analogWrite(4, random(0, 255));
        analogWrite(5, random(0, 255)); 
    }  
  }

}