Arduino

I want a LED to light up if a LDR detects that the light level has dropped.  I have another LEd running at the same time that shines on the LDR at 540 nanometres.  I want it to detect if the light level drops below 500 nanometres and if so turn on a REd led and if it is above 500 nanometres to turn on a green LED.

int LDRValue = 0; //result of analog pin
int ledPin = 13;

void setup(){
  Serial.begin(9600);//sets a serial port for communication
 pinMode(ledPin,OUTPUT);
}
void loop(){

  LDRValue = analogRead(LDRpin);//read the value from LDR
  
  Serial.println(LDRValue*12.5); //print to serial port
   digitalWrite(ledPin,HIGH);
  delay(200);
  
}

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