Make a relation between color and distance between objects' x coordinates

I'm trying to make the object to change color, it becomes whiter the closer it gets to the object "cubetarget", and i want to substract the x coordinates of the two objects and divide by 3, because the maximum distance between them is limited to 3, so maximum it'll be black, but I'm getting "Field "test.cubetarget" i never assigned to, and will always have its default value null". How can i fix it? and is there a better way doing this relation?

*Script's name is test.cs.

using UnityEngine; using System.Collections;

public class test : MonoBehaviour {

 GameObject cubetarget;
 public Renderer rend;
 float r; 
 float g; 
 float b; 

 void Start () {
     GameObject cubetarget =  GameObject.Find("cubetarget");
     rend = GetComponent<Renderer>();
 }
 
 void Update () {
             r= (Mathf.Abs((gameObject.transform.position.x - cubetarget.transform.position.x)/3));
     g= (Mathf.Abs((gameObject.transform.position.x - cubetarget.transform.position.x)/3));
     b= (Mathf.Abs((gameObject.transform.position.x - cubetarget.transform.position.x)/3));
     rend.material.color = new Color(r,g,b,1);
 }

}