When I picked up my car at the factory there were 3 grey cars. Two looked lighter, more dolphin grey. One was darker / more metallic. I said to myself I sure hope that's my car, It looks way better! And quick check of the wheels confirmed it was mine!
The Tesla employee I asked about it said all the metallic colors come out slightly differently with each batch, due to small volumes. Not sure I buy it, as I assume my car was in the same batch as the other two people picking up their car on the same day. Either way, definitely different color. I've parked next to a dolphin grey Model S a number of times, both around town and at Superchargers, and the difference is pretty clear. I see way more of the metallic / dark grey ones now than I did a year ago when I got my car, so maybe more consistent with the paint?
To add to the confusion, my grey also looks less metallic when it is dirty, but the dirt doesn't show up much, just hides the metallic-ness.