Every manufacturer has the original paint codes. They were similar to the codes on new car keys. That code does not actually tell you how the new key is to be cut but in a code book provided to dealers/ key shops, the code gives them the key blank info, the number and depth of each notch, and whether it is two sided cutting or not. The paint codes would cross over to the paint shops to tell them which pigments to use and in what amounts. They were mixed by weight. Problem is that paints have changed greatly. Some pigments deemed dangerous, to whatever. If you find the original formulas, that does not help unless you have access to the same pigments that the formulas were given.
Granted, as Tim mentioned, if you go back to the manufacturer, they have kept up better on crossing over pigments to newer paints. And exactly as he stated, the local paint vendors do not want to mess with calling Dupont, Ditzler, to whoever. Some car clubs such as the VCCA will use their "power in numbers" to get an old color registered as a "National Fleet Color" which gives it priority for being formulated in modern paints.
Outside that, some paint stores have an electro spectrometer which will scan the color and formulate it with modern pigments. It will also even list if a modern color is close. I had the transport blue and both of the interior blues scanned for my truck. I recall one of them came up real close to a BMW color. All that being said, A guy who I trusted as an excellent body and paint guy told me that the scanned colors leaned towards a shade darker or lighter, I don't recall which, that told me as I believe Denny mentioned, they may need to be tinted by eye to satisfy the purist.
I hope this helps.
Spectrometer info
