How to click in screen and change color in my sphere?

I have an sphere which change color if I click on it. But i need that user change the color of the sphere if he click screen, not in sphere.

Script change color :

public Material[] materials;
public Renderer rend;

private int index = 1;

// Use this for initialization
void Start () {

	rend = GetComponent<Renderer> ();
	rend.enabled = true;

}

public void OnMouseDown() {
	if (materials.Length == 0) {
		return;
	}
	if (Input.GetMouseButtonDown (0)) {
		index += 1;
	
		if (index == materials.Length + 1) {
			index = 1; 
		}
		print (index);

		rend.sharedMaterial = materials [index - 1];						
	}
}

public Material materials;
public Renderer rend;
private int index = 1;
// Use this for initialization
void Start () {

     rend = GetComponent<Renderer> ();
     rend.enabled = true;
 }
 void Update() {
     if (materials.Length == 0) {
         return;
     }
     if (Input.GetMouseButtonDown (0)) {
         index += 1;
     
         if (index == materials.Length + 1) {
             index = 1; 
         }
         print (index);
         rend.sharedMaterial = materials [index - 1];                        
     }
 }

Very good! Thanks!