Can't go wrong with black, but it blends in with every other phone in the world. Samsung color options are seldom trend-setting, but I'm hoping there's something unique for the S9, like the coral blue with gold trim on the Note7.
The copper Nokia 8 would be a stunning finish to copy.
I wouldn't mind going back to a white Galaxy, However being that they now put a black front on all of them, with a case on mine i wouldn't see anything different from what i see now.
It's probably to maximize the "bezel-less" effect. I personally like that it's all black on the front. I'm good with just seeing the metal trim and back as far as "color." I think most flagship devices are all black up front these days, even those that aren't black on the back or bezel-less.