How can I access a component in a prefab?
I have this script on a GO

public Transform prefab_Shadow;

public void SpawnShadow ()
{

	Transform newShadow = Instantiate (prefab_Shadow,transform.position,transform.rotation) as Transform;
	newShadow.transform.parent = gameObject.transform.parent;

So, I need to access the projector on this prefab (prefab_Shadow)
Thanks for any help!

Capture your instantiated reference as a GameObject, then use GetComponent, like so:

public GameObject prefab_Shadow;

GameObject newShadow = Instantiate (prefab_Shadow,transform.position,transform.rotation) as GameObject;

Projector projector = newShadow.GetComponent<Projector>();