im trying to make it if a specific item collides with a cube it flashes red but it keeps comming up with an error

It is impossible to call a yield outside of a co-routine to my knowledge. You might want to remove the following code from OnCollisionEnter and put in in a coroutine:

 yield WaitForSeconds(0.5); 
 gameObject.renderer.material.color = Color.grey; 

Also, yield WaitForSeconds() is javascript syntax, in c# it would be done as the following:

 yield return new WaitForSeconds();

I also think you have made some mistakes with the parentheses of the if-statements in OnCollisionEnter.

Using all of this information, you should end up with something like this:

 using UnityEngine; 
 using System.Collections; 
  
 public class swordCollideWithEnemybasic : MonoBehaviour { 
  
 private int enehealth = 0; 
  
 // Update is called once per frame 
 void OnCollisionEnter (Collision hit) { 
   if (enehealth <= 0) { 
     if (hit.gameObject.name == "sword") {
       gameObject.renderer.material.color = Color.red;
       StartCoroutine(turnGreyDelayed());
     }
   }
 }
 
 //Coroutine for setting color back to grey after 0.5 seconds
 IENumerator turnGreyDelayed() {
   yield return new WaitForSeconds(0.5f); 
   gameObject.renderer.material.color = Color.grey; 
 }

As @josh-naylor stated and is indicating with his comment, your OnCollisionEnter method is mal-formed, and i'm also going to say your if statements are not blocked correctly. If you don't use curly braces, an if statement will only execute the next line of code if it the expression is true.

 using UnityEngine; 
 using System.Collections; 
  
 public class swordCollideWithEnemybasic : MonoBehaviour { 
  
 private int enehealth = 0; 
  
 // Use this for initialization 
 void Start () { 
  
 } 
  
 // Update is called once per frame 
 IEnumerator OnCollisionEnter (Collision hit) { 
   if (enehealth <= 0) {
     if (hit.gameObject.name == "sword") {
       gameObject.renderer.material.color = Color.red; 
       yield return new WaitForSeconds(0.5); 
       gameObject.renderer.material.color = Color.grey;
     }
   }
 } 

The method in c# yielding the waitforseconds needs to return IEnumerator/IEnumerable.