Making a bullet invinceble

Hi!

I am trying to make the enemy bullet invincible and kind of pass right through the player bullet (Kind of enemy bullet eating the player bullet on it's path). I have 5 different bullets that both the player and enemy can shoot. YellowBullet, RedBullet, BlueBullet, GreyBullet and PurpleBullet each with a tag with their name.

Right now in my script I tried to set up a Boolean for the OnTriggerEnter method but not sure how I would make only the player bullet destroy and make the enemy one survive and pass through.

Here is my bullet script:

 using UnityEngine;
 using System.Collections;
 
 public class Projectile : MonoBehaviour {
 
     public float damage;
     public bool willDestroy = false;
     
     public float GetDamage(){
         return damage;
     }
     
     public void Hit(){
         Destroy (gameObject);
     }
     
     public void DestroyBullets(){
         willDestroy = true;
     }
     
     public void NullDestroyBullets(){
         willDestroy = false;
     }
     
     
     void OnTriggerEnter2D(Collider2D collider) {
             if(willDestroy = true){
                 Destroy (gameObject);
                 NullDestroyBullets();
         }
     }
 }

Player Script:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class PlayerController : MonoBehaviour {
 
     
     // Player Bullets
     public GameObject bullet;
     public float bulletSpeed;
     public Projectile invincibleBullet;
     
     
     // Player Health Slider
     public  static float currentHealth = 50f;
     public  static float maxHealth = 50f;
     private Slider playerHealthSlider;
     
     
     
     void Start(){
         playerHealthSlider = GameObject.FindGameObjectWithTag ("PlayerHealthSlider").GetComponent<Slider>();
         playerHealthSlider.value = calculateHealth();
     }
     
     void Update () {    
         playerHealthSlider.value = calculateHealth();
                 
         // (Input.GetMouseButtonDown(0) && GUIUtility.hotControl==0)
         if(Input.GetKeyDown(KeyCode.Mouse0)){
             GameObject playerBullet = Instantiate(bullet, new Vector3(0f,-13.5f,0f), Quaternion.identity) as GameObject;
             playerBullet.rigidbody2D.velocity = new Vector3(0,bulletSpeed,0);
             playerBullet.transform.parent = transform;
         }
         
         if(currentHealth <= 0){
             Destroy (gameObject);
         }
     }
     
     float calculateHealth(){
         return currentHealth / maxHealth;
     }
     
     public void TakeDamage(){
         currentHealth--;
     }
 }

And Enemy Script:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class RedEnemyScript : MonoBehaviour {
 
     // Reference.
     public EnemySpawner spawnScript;
     public EnemyAttackScript enemyAttack;
     public RedXpScript redXp;
     
     // Health.
     public float currentHealth;
     public float maxHealth;
     
     // Bullets.
     public Projectile invincibleBullet;
     public GameObject bullet;
     public float bulletSpeed;
     
     // Slider.
     private Slider enemyHealthSlider;
     private SliderColor healthColor;
     
     void Start(){
         
         // Enemy Health SLider
         enemyHealthSlider = GameObject.FindGameObjectWithTag ("EnemyHealthSlider").GetComponent<Slider>();
         enemyHealthSlider.value = calculateHealth();
         // Slider Color
         healthColor = GameObject.Find("Enemies").GetComponent<SliderColor>();
         healthColor.RedColor();
     }
     
     void Update(){
         enemyHealthSlider.value = calculateHealth();
         FullHealth ();
         if (currentHealth <= 0){    
             enemyAttack.ResetCountdown();
             redXp.XpIncrease();
             Destroy(gameObject);
             spawnScript.Spawn();
         }
     }
     
     float calculateHealth(){
         return currentHealth / maxHealth;
     }
     
     void FullHealth(){
         if(currentHealth >= maxHealth){
             currentHealth = maxHealth;
         }
     }
     
     public void Attack(){
         invincibleBullet.DestroyBullets();
         GameObject enemyBullet = Instantiate(bullet, new Vector3(0f,36.5f,0f), Quaternion.identity) as GameObject;
         enemyBullet.rigidbody2D.velocity = new Vector3(0,bulletSpeed,0);
         enemyBullet.transform.SetParent (GameObject.FindGameObjectWithTag("Enemies").transform, true);
     }
     
     
     void OnTriggerEnter2D (Collider2D collider){
         Projectile bullet = collider.gameObject.GetComponent<Projectile>();
         if(collider.gameObject.tag == "YellowBullet"){
             currentHealth = currentHealth - bullet.GetDamage();    
             bullet.Hit ();
             enemyAttack.EnemyAttack();
         } else if(collider.gameObject.tag == "RedBullet"){
             currentHealth = currentHealth + bullet.GetDamage();    
             bullet.Hit();        
             enemyAttack.EnemyAttack();
         } else if(collider.gameObject.tag == "BlueBullet"){
             currentHealth = currentHealth - bullet.GetDamage()*2;
             bullet.Hit ();    
             enemyAttack.EnemyAttack();
         } else if(collider.gameObject.tag == "GreenBullet"){    
             currentHealth = currentHealth - bullet.GetDamage()/2;
             bullet.Hit ();        
             enemyAttack.EnemyAttack();
         } else if(collider.gameObject.tag == "GreyBullet"){
             currentHealth = currentHealth - bullet.GetDamage()/2;
             bullet.Hit ();
             enemyAttack.EnemyAttack();
         } else if(collider.gameObject.tag == "PurpleBullet"){    
             currentHealth = currentHealth - bullet.GetDamage()*2;
             bullet.Hit ();
             enemyAttack.EnemyAttack();
         }
     }
 }