How i can make a function call and then after 15 sec another function call during this function call the first one stops.And this process keep repeating again and again.

I have a 2D game. I am in a problem. My game has three levels. Actually i have to cars. one is white other is yellow. And there are two types of incoming circles white and yellow. and currently my game is running perfectly. In level one, when yellow car collides with yellow it increases score.same for white car when it collide with white circle score increases. In level 2 everything is same except the factor that cars have to collect opposite color circles. Now in level 3 what i want is that when it starts the cars have to collect same color circles like in level one and after 15 sec cars have to collect opposite color circles like in level 2. and after 15 sec again cars have to collect same color circles like level oneā€¦ And this all keeps repeating. really need help its urgent guys. Thank you in advance.
This code is for level 1

void OnTriggerEnter2D(Collider2D	other){
	


		if (other.CompareTag ("white") && car == "white") {
			Destroy (other.gameObject);
			SoundManager.instance.PlaySingle (circle);
			count = count + 1;
			SetCountText ();
			}
		if (other.CompareTag ("Yellow") && car == "white") {
			Application.LoadLevel (2);
		
		}


		if (other.CompareTag ("Yellow") && car == "yellow") {

			Destroy (other.gameObject);
			SoundManager.instance.PlaySingle (circle);
			count = count + 1;
			SetCountText ();
			}
		if (other.CompareTag ("white") && car == "yellow") {
			Application.LoadLevel (2);
			 
		
		}
	
	}

This is for Level 2

void OnTriggerEnter2D(Collider2D	other){



		if (other.CompareTag ("Yellow") && car == "white") {
			Destroy (other.gameObject);
			SoundManager.instance.PlaySingle (circle);
			count = count + 1;
			SetCountText ();
		}
		if (other.CompareTag ("white") && car == "white") {
			Application.LoadLevel (2);

		}


		if (other.CompareTag ("white") && car == "yellow") {

			Destroy (other.gameObject);
			SoundManager.instance.PlaySingle (circle);
			count = count + 1;
			SetCountText ();
		}
		if (other.CompareTag ("Yellow") && car == "yellow") {
			Application.LoadLevel (2);


		}

	}

Private bool isOpposite = false;
Private float counter = 15.0f;

Private void Update ()
{
      counter -= Time.deltaTime;

      If(counter <= 0)
      {
            counter = 0;
            isOpposite = !isOpposite;
      }           

      If(isOpposite)
            // Do the opposite colours
      Else
            // Do the normal colours
}