boost.cs

// >>>>>>>>>>>>Player Controller.cs<<<<<<<<<<

//...	

	public void BoostSpeed(float duration, float factor){
		StartCoroutine (BoostCoroutine(duration,factor));
	}

	private IEnumerator BoostCoroutine(float duration, float factor){
		boostFactor *= factor;
		rend.materials [2].color = boostColor;
		yield return new WaitForSeconds (duration);
		boostFactor /= factor;
		rend.materials [2].color = baseColor;
	}
}

//<<<<<<<<<Boost.cs >>>>>>>
public float duration = 5f;
	public float factor = 1.6f;
	PlayerController pController;
	void Start () {
		pController = GameObject.FindGameObjectWithTag ("Player").GetComponent<PlayerController> ();
	}
	
	void OnTriggerEnter(Collider col){
		pController.BoostSpeed (duration, factor);
		Destroy (gameObject);
	}