Which colors do you use?

The question about the Enterprise-D's colors got me thinking of a potential survey subject.

Sometimes the appearance of a model on-screen is different from the actual color of the miniature. For example...

The Enterprise-D looks gray most of the time, but the studio miniature is actually bluish. Some studio head decided the blue was too different and had the tone "bleached out" in post-production.

The Vulcan Shuttle in the first Star Trek looks gray, but was actually painted in multiple sandy pink tones. The change was made for the same reasons as with the Ent.-D.

The TIE fighters in the original Star Wars were a light blue, but seemed white or light gray on-screen. Unlike the above examples, this was because ILM's builders had accidentally mixed a "non-photo" blue, visible to the eye but unable to register on film.

There are also cases where the same ship seems to be different colors at different times. Luke's X-Wing, for example, seems to be gray much of the time, but the full-size prop looks more like it's a flat metallic finish, like bare weathered aluminum ( check where he's gearing up to leave Hoth and the raising-out-of-the-swamp shots in Empire ).

So my question is this. How do you choose which colors to use? Do you base it on the actual studio miniature's colors, or on-screen appearance? If the on-screen appearance varies for the same ship, how do you decide which reference to use?

Anyone?