New images have come out showing the iPad mini 2 in aluminum as compared to the space grey color. As you can see it's a relatively subtle difference between the two options. Obviously, the space grey is darker, but it also looks like the Apple logo on the back has been shifted to black to complement the space grey. In fact, it looks like the Apple logo on the standard aluminum could be space grey itself.
There is still no word on if the new champagne color of the iPhone 5s will be making the move to the iPad or iPad mini. We should be about a month away from finding out though. The rumors are that Apple will be announcing the new iPads next month, and some are putting the date at October 15th.