Turn on a LED when an other LED turn off
First, make sure the LEDs are connected correctly: I presume the red LED is connected to whichever pin LR is, and the green LED to LV.

According to your diagram, your LEDs will be on if the output is HIGH, and off if the output is LOW.  Your code sets LR to high, but then immediately to LOW again.

Here's a fixed (I hope) verson of your loop():

void loop()
{     
  readSerial();
  if (received_code == 200){
    // Turn the red LED on
    digitalWrite(LR,HIGH);

    // Turn the green LED off
    digitalWrite(LV,LOW); 
  } 
}




Unrelated to the problem you're having, but worth a mention anyway:

There's nothing in your code to stop you overrunning buf if you get a longer input than you're expecting.  If this happens you'll get unpredictable results as you trash over whatever happens to be in that part of memory.