In the case of Colony, that's more than metaphor. The Earth has apparently been invaded - we've become a colony, or maybe a series of colonies would be a better way of putting it - but by whom? In episode 9, we were treated to what looked like a robot arm. Or, was it really just some sort of cybersuit? Concealing ... what? An alien? Or maybe a human?
In the season finale, we get to see more or less of this complete exterior of the alien - but with no further indication of what was inside it. This lack of revelation is a clearly a deliberate decision on the part of the show's creators.
One of them, Carlton Cuse, has a history of television series with a lack of sufficient revelation - in fact, his series (with Damon Lindelof), Lost, is justifiably the most celebrated series with insufficient revelation throughout its narrative. And this led to a profoundly uneven story, with some of the best episodes and even seasons ever on television in any series, and also some of the worst.
The frustrating thing about Colony is that it's doing a fine job down here on Earth. I'd have no complaints at all if the story was not about a presumed alien invasion, but maybe a fascistic revolution in the near future right here in the United States. (Hmm... looking at what's going on in the Republican Party these days, that's all too plausible.) But everyone in the action keeps telling us that's not what's going on.
And that being the case, I really wish we'd see a little about the invaders. Well, I'll give this show at least one more season to see what it does - and maybe that was the idea in the first place.
See also Colony 1.1: Aliens with Potential ... 1.2: Compelling ... 1.5: Questions ... 1.6: The Provost ... Colony 1.7: Broussard ... Colony 1.8: Moon Base and Transit Zones ... Colony 1.9: Robot Arm
not exactly aliens, but strange enough ... The Silk Code