NullReferenceException: Object reference not set to an instance of an object

Ok so i made a script it goes

 function OnMouseEnter(){
     Renderer.Material.Color=Color.red;
 }
 
 function OnMouseExit(){
     Renderer.Material.Color=Color.white;
 }

, And i get an Error Saying

`NullReferenceException: Object reference not set to an instance of an object`

I'm not sure why i get this. If you could help please do so. Thanks!