Went and compared the colors at BB yesterday. They had both the S8 and S8+ in all three colors. The Orchid Gray looks more orchid than grey to my eyes. It reminded me a lot of the Black Sapphire that the S6 Edge came in, which in my opinion usually looks more sapphire than black. I really wished the Orchid looked more gunmetal, as some had suggested, but I didn't see this at all. What I didn't realize prior to going in was that the bezel (the metal part around the edges, not around the screen) has a purple tinge to it as well.
After seeing the black in person, I think it looks better in photos than in reality. I agree with previous comments that it looked (and felt) more plasticy than the other two colors. If it had been a more metallic black or like a brushed look I would have liked it better. As it is, it reminds me of someone who plasti-dipped all the chrome on their car to a gloss black; not just in color, but texture as well. I had pre-ordered an S8+ in black, but cancelled and re-ordered in silver. Not my favorite color of all time, but I liked it most out of the three. I know my wife will snatch up an Orchid Gray as soon has her current phone is paid off.