IMO, what color you use depends on what you're modeling.
If you're re-creating a true studio-scale miniature, then obviously your goal should be to find the right color to match the original models. In this case, you must have decent pics of the models themselves, shot with as little extra light as is necessary to take the picture.
If you're building a smaller scale kit, one which recreates the vehicles as seen in the film, then your goal should be to match the way those models look on screen. In this case, you must consider both scale-effect, and "film-effect," by which I mean the amount of light used to film the miniature using motion control cameras (
a whole lot!) and the contrast wash-out effect of film compositing.
There are various shades of "TIE-Fighter Blue" used in the three Star Wars films. The models in ANH appear almost pure white-gray on screen, even though they are much more of an intermediate blue-gray uder normal lighting conditions.
Check out
Phillipe's closeups of Vaders TIE for a good look at the blue-gray used for ANH.
Later models built for ESB and ROTJ were much bluer.
Here are Phillipe's pics of what I believe is a later TIE, probably from ESB.
As for what color to use, that's always a subjective thing and I'm not aware of any color wheel "canon" for Star Wars models, like you often find for Star Trek models and props.
I just mix my own colors because I've never found a good out-of-the-bottle hobby paint match for the colors the ILM guys used. I usually start with the closest match I can find, and then I lighten whith light gray or pure white, darken with dark gray or pure black, and add little bits of red, yellow, and/or blue as necessary. To see how the paint will look when dry, I'll brush about a one-inch square on a sheet of styrene for each test color mix. Under each square I write the formula used to mix that sample ("1 part gray, 2 parts white, etc."). Finally, I pick the color I like best after each test mix has dried.
Somewhere, I've got a styrene card with a perfect ANH blue-gray match in Poly-Scale paints, but we moved this year and it's still packed away in a box in the attic.
This method of color-matching is time consuming, but it's worth the effort, IMO.