[ArkhanTheBlack]

Well, it's indeed grey(checked it with paint). I guess the redish appearance comes from the red panels in some of the windows.

I'd also keep it diplomatic. Just use a good looking background for the units. ;-)
However, I wonder, if the unit images look so much better with black background, then why wasn't it used for the unit images in the description windows?!
Actually I like grey better than the pitch black. Black is not a natural background.

However, the actual point wasn't the background color of the units but that it was used for the whole screen.
Just keep it in a combat window and you have both, good unit contrasts and a nice background.