They could've used the same colors in more than one year model, that's my guess.

The easiest way to find the correct code is to get a hold of a 1990 service manual. I had the same issue you're having and the only way I was able to confirm the code was with the manual, on mine ('87) it list the color codes available for the model year on page 7. Here's one for a good price, and if you're restoring this truck I'm sure having this will be helpful down the road
https://www.ebay.com/itm/2-Volumes-1...EAAOSw3x1dgCQA

Once you know the code, one of the two paint companies mentioned before will be able to mix it, I had a good experience with automotive touch up.