int RGBr=21; 
int RGBg=22;
int RGBb=23;  
void setup(){
  pinMode(RGBr, OUTPUT); 
  pinMode(RGBg, OUTPUT); 
  pinMode(RGBb, OUTPUT); 
}
void loop(){
  digitalWrite(RGBr, HIGH);
  digitalWrite(RGBb, LOW);
  digitalWrite(RGBg, LOW);
  delay(1000);
  digitalWrite(RGBr, HIGH);
  digitalWrite(RGBb, LOW);
  digitalWrite(RGBg, HIGH);
  delay(1000);
  digitalWrite(RGBr, LOW);
  digitalWrite(RGBb, LOW);
  digitalWrite(RGBg, HIGH);
  delay(1000);
  digitalWrite(RGBr, HIGH);
  digitalWrite(RGBb, LOW);
  digitalWrite(RGBg, HIGH);
  delay(1000);
}