Al, I mainly do what Monty suggested, I use Google first to try and find as many pictures of part or parts in question. I also use all the Model Master metalizer paints and if you use an airbrush Alclad has a wide array of metal colors too. I also use flat, semi gloss and gloss black, and also have a few different colors of silver and aluminum, every manufactuer's colors are different. What I do is take some Plastic spoons and spray them with different silvers or black or whatever color and then take a sharpie and write on the handle what it is then you have a color reference to go by and you can kind of match your color spoons to the picture of the part you want to paint.
I also mix my own custom colors for different things especially older stock carbs, they have that greenish gold color. I usually take some yellow, black, and gold paint and tweak it until I get it to where my eyes say it matches. So yes it all comes down to pretty much what Monty said, mostly an experiment to get as close as you can with what's available. Hope this helps.