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Here is the generic info packet from the GM Heritage Center Collection; this will get you the RPO code and noun name and we can convert to the WA number from there.
Depending on how curious you are, you can send your VIN (and $50) to the GM Heritage Center and they can pull your original invoice, which would include build timing, dealer info, original option content and pricing.
90U is the paint RPO. Referring back to the chart I posted earlier reveals that to be Gray Steel Metallic. Quick internet search shows that to equate to WA9191. I happen to know the paint type in 1987 to be “high solid enamel”.
(“U”= “Upper”, or primary. “L” = “lower”, or secondary)