Exit function when conditions met

Projects Programming
2 
void loop() {

  randomSeed(analogRead(5));

randomSeed() should be called once, not on every pass through loop().

  while(endTimer - startTimer <=n){
   
 
   random_LED = array_LED[random(0,3)];
 
   delay(random(0,2000));
   digitalWrite(random_LED, HIGH);
   
   
   switch(random_LED){

    case LEDpin_RED:
     
    LED(BUTTONpin_RED,LEDpin_RED);

When you return from LED, you return to this while loop. The while loop has no idea that you want to quit.

LED needs to return a value, and you need to test that value, to decide whether or not to break out of the while loop.