Man, that's the million dollar question! One of the biggest debates since the "6 or 7 grips" thing, or the "heiland vs MPP" thing, or the ".9mm vs .45 cal" thing. Gray is one of THE most pain-in-the-butt colors to nail, due to it's ability to absorb lighting, room colors etc. I had the same trouble with my star destroyer...the studio miniature appeared to be half a dozen different colors in the three films!
Testors Light gull gray is what I used on my Polar Lights TOS Enterprise, but it's really anybody's guess
Try this link....it should shed some light on "ONE OF THE GREATEST MYSTERIES KNOWN TO MAN!!"(echo-echo-echo)
Dude, your Star Destroyer blew me away. That was one of my most favorite threads. Yeah, I did a duck and run for cover after posting this and I have been cruising Cult's site as well as Starship Modeler for info. I have a sweet 22" TOS model, complete with lights and great nacelle swirl effect made by Moffeaton. It really need a new paint job, so I'm going to hire a local model maker to spruce her up. I just want to give him as much direction as I cna. I'm gonna go with light gull for the hull, it's white now and I just don't like that look.
First link has numerous pics of the original models (including the various renovations the 11 footer has had). In most of the pictures, the ship ranges from looking light gull grey to white (in many of the 3 ft model shots).
Second is a Thomas Models (who made the kits for PL) page showing step by step how they built up each version of the kit for press pictures. They ended up using white mixed with a bit of silver on theirs.