When it was decided to actually show the Klingon Battlecruiser on-screen, Paramount commissioned AMT to build two filming models. One model was delivered as soon as possible, so filming could start, and the other was used in the production of the model kit, which is exactly one-half the size of the filming models. Once AMT was finished using the second model for the pantographing process, they delivered it to Paramount.
It was never used for filming and sat in Roddenberry's office, on a stand, until the end of the series. Roddenberry then gave it to someone and it changed hands several times before winding up in a Profiles in History auction in 2006. These photos are of that model, which was painted exactly the same as the filming model. It was always intended that this model was to be used for filming if something happened to the first model or they just needed another model in a shot.






It is now known that the original filming model was delivered to the Smithsonian with the paint scheme seen in these photos. But, when both models were delivered by AMT, they were only painted in a medium gray. Matt Jefferies added the gray-green to the first model and filming was started. When the second model was delivered, he updated the paint job on it and it then sat in Roddenberry's office until he moved out when the series ended.
When it was decided to do the 'Phase Two' series, the original filming model was brought back from the Smithsonian and repainted, adding a 3rd (darker green) color. It is not known if any tests were made of the model in this paint scheme.
When 'Star Wars' came out and proved to be a huge success, Paramount scrapped the idea of a new TV series and started developing a theatrical feature. It was decided the existing model for the D-7 would not be practical to use for the big screen, so a mold was made of the ship and castings were used for reference to build a 2X model.
The D-7 was then sent to Ed Miarecki (I don't know if I spelled that right...) for reassembly and restoration before its return to the Smithsonian. It was painted using only a single gray and put back on display. We have to assume Mr. Miarecki was never told what the colors were supposed to be and that because the ship appeared to be a single, bluish-gray on screen, he got as close to a 'correct' color as he could.

