John,
I've been putting together a 1/650 scale Enterprise TOS model for the "Hollywood GB" and have been doing some research into colors for the TOS Enterprise (I guess that's what you're looking for).
Now the Polar Lights 1/350 scale model box recommends XF-12, or IJN Grey, although it says to lighten it somewhat. If you have an airbrush, you can try that.
Right out of the can, I think this color is closest to the original color of the Enterprise, though I'm kinda iffy on that.
http://www.myperfectcolor.com/en/color/75830_Walmart-96321-Concrete
I intend to do another "accurized" version of the old AMT model, so this is the color I will be ordering for that project.
(As a side note, I do have the 1/350 scale version but I'm not ready to tackle that one yet!)
I've also been looking online for any original photos of the filming model that were taken back when the model was new and relatively untouched, even though the model went through revisions even as the show was being filmed back in the 60s. A few minor differences in the Polar Lights model and the first season filming model (after the revisions from the pilot and 2nd pilot) are the reason I've been looking.
First, these are probably the best close-up shots I've seen of the actual filming model as it sits in the Smithsonian right now, though colors are not quite clear as the model was shot with flash and through the plexiglass, but for detail they are great.
http://www.mjtsc.pwp.blueyonder.co.uk/
For photos of the model during filming, you can try this site.
http://www.startrekpropauthority.com/2008/05/uss-enterprise-original-series-11-foot.html
Lots of interesting photos here.
Not sure if this will be of any use to you, but it's awfully good information for ST modelers.
Gary
PS> And maybe this one. Forgot about it.
http://www.startrekhistory.com/models.html