While there are a few lists of the 3 digit codes floating around online, none of them are 100% complete. I made my own version by creating a garage with the same number of cars as in a Hex range and applying over a hundred codes (in numerical order) at a time, per edit and then writing down what colors they produced in the game. This way I could just change one digit per code each edit and speed things along substantially.
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This took me a few months as I recall, and in the end, I matched the ones not in other lists up with cars they belonged to or appeared to belong to in the game and made a short-list of just the unchipped or 'hidden' colors. Some had names in the game and some had 'No Name" (NN). For the no-name colors I looked up the real cars and real color names and wrote them in to make it easier to differentiate no-name reds, greens, yellows and blues from each other.
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When I was done, I cross-checked the no-names with Razerman to fix any errors I had committed and this list of "Hidden Colors" is the result. I have a photo album of the fifty or so most useful ones labelled with the names and codes. I was going to upload it to make it easier for everyone 2 years ago, but I was too burned out to get to it and just have been trying to find time ever since to go back and play in my box of GT5 crayons again. What sounds like a lot of work (it is!) I find to be quite fun and rewarding so, I'm look forward to doing some more fun stuff as soon as I can.
EDIT: It may be noteworthy to mention, Not every no-name color is a body color, a small percentage are stripe colors, bumper colors or even tail light colors. What ever was the last color modeled for that car. The digital artists always modeled the body color last for chipped cars, but this was an unnecessary step for unchipped cars so they didn't always bother doing it that way.
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For this reason some of the colors listed are not exact matches to the car in the game because they may only match half of a two-tone color scheme or not have the top layer or clear coat that is hard coded to the individual car that gives the final appearance of a chrome-color for example. But they are still useful colors none the less.