I didn't think the platinum looked all that good until I saw it in person. Then, I wanted it, but the store only had black (and my wife had decreed I had to walk out with a phone that day). I think I actually would have gotten the orchid gray GS8 if the G6 hadn't been 50% off. I just really didn't want yet another black phone, and the fingerprint visibility was another factor. But the $$$ won out and I got the black G6.
I have to say, though, that I am glad I got the black. The all-black front looks better, especially given the little black border between the actual display and the silver bezel. I use the black theme with as many black backgrounds as I can (hate being forced to use a white background!) and the visual effect is striking.
Ultimately, I don't think you can go wrong with either, they both look great, just in different ways.