Could it be that if a director wants someone who will "look ethnic" (i.e. the audience can "see" his or her ethnicity), the director will pick someone "obviously" ethnic, i.e. darker - like Naveen Andrews?

That's what I'm thinking; as you said, Abraham or Shalhoub can and do play ethnic roles, but frequently just get cast as "Americans." We (white Americans?) need "something else" to differentiate "them" from "us."