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.
no clue why they made it purple, probably just a gameplay issue...to identify them easier.
but i'm just guessing.
maybe there was an explanation for it, because i'm pretty sure one year ago you couldn't tell them appart from real Starfleet vessels. I think they got a redesign right before F2P had hit.