change all materials color back to original

I have different materials with different color for each gameobject. manage to change all materials color but didn’t manage to revert them back to their original state. the objects revert back to full black color. cant find the cause of this. here’s an example script:

private Color originalColor;
void Start()
{
   foreach(Material mat in this.gameObject.GetComponent<Renderer>().materials)
   {
       mat.color = originalColor;
   }
}
void OnCollisionEnter(Collision other)
{
   foreach(Material mat in this.gameObject.GetComponent<Renderer>().materials)
   {
       mat.color = color.grey;
   }
}
void OnCollisionExit(Collision other)
{
   foreach(Material mat in this.gameObject.GetComponent<Renderer>().materials)
   {
       mat.color = originalColor;
   }
}

Are you initializing the originalColor ? if not it most likely is black.