Most frequently used colors?

For me, it depends on the model I'm building. Of course I use black and white (both gloss and flat) all the time, just like everybody else, but for other models, it varies. For the MiG-21, I used a lot of USSR interior blue green in the cockpit, then for the exterior of the plane, I made a special color I called "Arizona MiG Silver" (because the plane I was modeling used to be at the Champlin Fighter Museum in Mesa, Arizona). Because I have no place to airbrush in my 2nd story apartment, I had to brush paint an entire 1/48 scale MiG-21; you can't buy Arizona MiG Silver in a spray can! My most recent finished project was a 1932 Chrysler Imperial; on that one, I used a lot of black and steel for the engine compartment and chassis (some of it guesswork; this was an ancient kit with virtually NO painting instructions and when my references showed the engine compartment, the pictures were in black and white); a lot of a custom-mixed brown for the interior, and Testor's gloss brown and light ivory from a spray can for the body (with flat sand for the convertible top). Right now, I'm painting the interior tub of my tornado chase vehicle; after studying Jeep Cherokee brochures and ads, I decided to paint it "quartz," a pretty shade of gray with a silver tint. I had to mix this color, of course; it's a little dark but it matches the silver-gray fabric I'm using as "carpet" in the interior. (The fabric is a little too dark, but it was the closest I could find to what I want.)

Most used? Blacks & Whites used by themselves or mixing with other colors to shade up or down. Actually, airbrush thinner for enamels (testors by the pint can) and lacquer thinner for airbrush clean-up & paint brush clean-up. Get the lacquer thinner at the hardware store by the quart or half=gallon. Saves $$$.
G.L.

For armor I would say I use these more often in large quantities that the others:

1) Olive Drab
2.) Dark Yellow
3.) Dark Green
4.) Red Brown
5.) Flat Earth
6.) Metallic Grey
7.) Gun Metal
8.) Flat White
9.) Flat Black
10.) Buff
11.) Field Grey
12.) German Grey

I keep my paints in boxes marked for what country they are for (Ger, Br, USA). I keep one box marked general. This is those colors most likely to be found on all aircraft, i.e. black, white, aluminum, steel (graphite), gun metal, zinc chromate green, zinc chromate yellow and a selection of basic red, yellow, and blue. Most of the painst I use are Humbrol and Model Master. Just in passing I use Dupont Fast Dry Enamel Reducer #3812S. I've never had a problem with compatibility, buy it by the gallon (which lasts about a year and a half, even airbrushing and is a heck of a lot cheaper than the little bottles of thinner in you local hobby shop)

I find myself buying RLM-02 and RLM-65 in larger and larger quantities.

I have more shades of gray...

You would need white for gauges and lights, aluminum and chrome for landing gear and red and green for navigation lights