color is not changing in enum code

I can't get this script to work, I am trying to get an object to change color based on what I drag into it. I don't get errors when I build the code but I do when I play the game and try to drag in the objects. Please help

(edit) Here are the errors I get

NullReferenceException: Object reference not set to an instance of an object CollisionAction.DoRed (UnityEngine.GameObject destroyObject) (at Assets/Scripts/CollisionAction.cs:48) CollisionAction.OnCollisionEnter (UnityEngine.Collision other) (at Assets/Scripts/CollisionAction.cs:29)

NullReferenceException: Object reference not set to an instance of an object CollisionAction.DoBlue (UnityEngine.GameObject destroyObject) (at Assets/Scripts/CollisionAction.cs:65) CollisionAction.OnCollisionEnter (UnityEngine.Collision other) (at Assets/Scripts/CollisionAction.cs:26)

     bool activated;
 int value;

 private MeshRenderer wallRenderer;
 private Material wallMaterial;
 private CollisionProperty colPop;

 void Awake (){
     wallRenderer = GetComponent<MeshRenderer> ();
     wallMaterial = wallRenderer.material;
     colPop = GetComponent<CollisionProperty> ();
 }

 void OnCollisionEnter(Collision other){
     CollisionProperty cp = other.gameObject.GetComponent<CollisionProperty> ();

     if (cp != null){
         switch (cp.myProp)
         {
         case CollisionProperty.PropType.blue:
             DoBlue (other.gameObject);
             break;
         case CollisionProperty.PropType.red:
             DoRed (other.gameObject);
             break;
         case CollisionProperty.PropType.green:
             DoGreen ();
             break;
         case CollisionProperty.PropType.yellow:
             DoYellow();
             break;
         case CollisionProperty.PropType.goal1:
             DoG1();
             break;
         case CollisionProperty.PropType.goal2:
             DoG2();
             break;
         }
     }
 }
 
 public void DoRed(GameObject destroyObject){
     if (activated == false && (colPop.myProp != CollisionProperty.PropType.red)) {
         activated = true;
         Destroy (destroyObject.gameObject);
         wallMaterial.color = Color.red;
         colPop.myProp = CollisionProperty.PropType.red;
         StartCoroutine (countdown ());
     } else if (colPop.myProp == CollisionProperty.PropType.red) {
         //add more points because it is the same color
     } else {
         activated= true;
         Destroy (destroyObject.gameObject);
         wallMaterial.color = Color.red;
         //just changed color
     }
 }
 
 public void DoBlue(GameObject destroyObject){
     if (activated == false && (colPop.myProp != CollisionProperty.PropType.blue)) {
         activated = true;
         Destroy (destroyObject.gameObject);
         wallMaterial.color = Color.blue;
         colPop.myProp = CollisionProperty.PropType.blue;
         StartCoroutine (countdown ());
     } else if (colPop.myProp == CollisionProperty.PropType.blue) {
         //add more points because it is the same color
     } else {
         activated= true;
         Destroy (destroyObject.gameObject);
         wallMaterial.color = Color.blue;
         //just changed color
     }
 }
 public void DoGreen(){

 }
 
 public void DoYellow(){

 }
 public void DoG1(){

 }
 public void DoG2(){

 }
 IEnumerator countdown()
 {
     yield return new WaitForSeconds(5);
     activated = false;
     Debug.Log ("false");
 }

}