Why doesn't enabling and disabling not work?

I am a noob a programer but I can't find how to find a solution to this error:

NullReferenceException: Object reference not set to an instance of an object CTF.Changing_Color.ChangingFlagColor () (at Assets/Scripts/Changing_Color.cs:29) CTF.Changing_Color.Start () (at Assets/Scripts/Changing_Color.cs:16)

My Code:

 using UnityEngine;
 using System.Collections;
 
 namespace CTF {
     public class Changing_Color : MonoBehaviour
     {
         private RaycastHit hit;
         public GameObject Red;
         public GameObject Blue;
         public GameObject Neutral;
 
         // Use this for initialization
         void Start()
         {
             
         }
 
         // Update is called once per frame
         void Update()
         {
             ChangingFlagColor();
         }
 
     
 
         void ChangingFlagColor()
         {
             if(hit.transform.CompareTag("Blue"))
             {
                 Red.SetActive(false);
                 Blue.SetActive(true);
                 Neutral.SetActive(false);
             }
             if (hit.transform.CompareTag("Red"))
             {
                 Red.SetActive(true);
                 Blue.SetActive(false);
                 Neutral.SetActive(false);
             }
 
         }
     }
 }