Typically GM has a variety of places they like to stash the paint codes....... Glove box as well as drivers door are the most common. One thing is tried and true, You are looking for a four number that is preceded by the letters "WA". With your truck it should have two "wa" numbers being it is two toned.
Example - wa 8587 is a "medium blue poly" on a 83 truck
But as our trucks age (hopefully with grace) the tags get removed and damaged. So you can always go to an automotive paint store and most of the better ones have chip books that date back to the 70's. Then all you do is look up the year of your truck compare the different color chips to your trucks paint and then write down the correct code from the book. This will only work if it is an original color. If somebody has painted your truck with a custom color or resprayed the truck with anything other than what came on it, you will have to have the paint store do an "eye match".
They will take part of your truck (usually the gas door or something small and start mixing until they build a color that matches. Usually it takes a day or two.
Good luck.
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