On the back of the compiled/revamped/reissue of Paul Newitt's booklets in digest form from CultTVMan press is a PMS 'color chip' of the original Enterprise color. It is much darker than you'd think, a concrete grey with a hint of green to it. (my description). Both the 11 foot and 3 foot studio models were painted this color, so to drop it to scale with the Ertl or Polar Lights kits, you might want to add an imperceptible amount of white. The reason the ship looked all those differing colors was two-fold. Partly because studio lighting and blue-screen affect, and partly because of the type of film used. It is enough to drive a monk to madness!
For one version I've made, I decided to go for a true TV appearance. I did it in camo gray, dusted it with silver, and then toothbrush-spattered it with a mix of both all over to simulate the grain and distortion of the filming process. It only looks "right" in person, in photos it looks dead white, if not a little dirtyish. It's ok for a shelf-warmer, I guess. :)
There are all kinds of sites on the internet with plans and illustrations-- but the best I've seen for brass tack detail of the studio model is the IDIC Page. Utilizing that, combined with the Newitt book, you will have a better representation of the 11-foot studio model in her heyday than even what resides in the Smithsonian today.
Oh, and if anyone ever wonders... the hull lettering DOES have a thin mustard/gold outline. I've held the actual unmounted replacement decals in my hand, and by gum, they have it. Should you try to replicate that on your model? Well, in actual size, it was 1/16 of an inch in width around the saucer top NCC-1701 letters, so if you have a laser printer than can make outlines 1/117 of an inch wide... DO IT!
Have fun building it, regardless of what path you choose. It is YOUR project.
George