Here's my take on it.
In the episode we saw, when they showed the ISS Enterprise, she was colored exactly the same as the USS Enterprise. The only thing that really was different was the signage. However, I also like the idea of what they did in ST:VOY "Living Witness" (which for all intents and purposes nicely shows a Mirror Universe Voyager and crew), with beefing up the weaponry. More visible weaponry to intimidate and terrorize.
Also in fan fiction, there was an awfully good story I read depicting a Mirror Universe Picard and crew where Picard retained the Borg armor (after he was recaptured by his people from the Borg) in order to more strike fear into the hearts of his enemies. Think he even retained the Borg shielding as well.
So, I'd not change the coloring too much.. maybe a bit darker. But definitely enhance the weapons prescence by maybe lighting the areas where the weapons are.. a dull red or orange at the phaser stripping, lighting on the photon ports, and for non-texture related ideas, maybe putting a few surface mounted phaser siege cannons and such on the hull. Mirror Universe weapons development would probably be ahead of the normal Trek universe, so adding quantum torpedos to a Galaxy Class ship would work. Shielding would be important. Remember, these ships would in some cases go in alone and lay siege to a planet.
That's just my two cents worth...