The color of the Terran Empire ships is way of base with canonism. This ships from mirror universe were not dark ships with glowing purple engines and surfaces. The ships in the shows looked exactly the same as the ships in this universe. In TOS, and DS9 the ship looked the same, just the uniforms and people style was different. Even in the Mutltiverse in TNG, the ships still look the same. I think the Devs went too far to make the Mirror ships different and now ships like Jem'Hadar ships. If they want to just change the material the ships are made, then just change ship material, but not so drasticly that the ships look like dark ships from a horrer scene.