Commissioning photo of the Seawolf - http://www.navsource.org/archives/08/08002102.jpg
I would link the picture directly but NAVSOURCE disallows hot links.
On most operational US SSNs the lower half of the hull is hull red. Many were delivered with the lower 2/3 (to the waterline) in red.
Note the photo, from the halfway point to the upper third (or quarter) -- a very dark charcoal gray. Top third - black -- almost a gloss black. The bow sonar dome is graphite reinforced plastic and is a slightly different gray. There is also a non-skid walkway which I've done by applying a flat coat patch on the topside hull color.
USS Connecticut (SSN-22) fitting out http://www.navsource.org/archives/08/08002201.jpg
USS Jimmy Carter (SSN-23) roll-out: http://www.navsource.org/archives/08/08002301.jpg
Note black underhull, note too the dark gray on the upper part of the hull