Here's a link to another supplier of quality automotive engine enamel. They do have the Buick Greens and Reds, but they don't mention the '63 Riv only silver/aluminum/grey/what evershade that really is. They sell a plain silver.
http://www.hirschauto.com/prodinfo.asp?number=EE-QUARTClassic Buicks sells a silver paint for the '63 Riv. Classic Buicks used to sell their part #PT-16 which was really AERVOE paint's part #500. That was a really good match. I orderd a new can of PT-16 from Classic Buicks. Before my Classic Buick order arrived, I contacted AERVOE about perhaps buying it directly from them and was told that part #500 was no longer produced. They sent me samples of their #575 - Dull Aluminum, and #570 - Universal Silver.
When the paint from Classic Buick arrived, I peeled their label off and found AERVOE part #575 under it. Classic changed the paint color they're selling as PT-16 but didn't change the part number.
In my opinion, the #570 - Universal Silver is a better match to the old #500 than part #575 - Dull Aluminum. BUT, there's no original paint on my '63 to match anything to. I can only compare to what I originally repainted with the #500.
Just for grins, I ordered a can of Buick Green from classic Buicks. When I peeled the label off of it, underneath was Dupli-Color part E1617: You're going to love this -
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FORD GREENIt looks like the old 1929 - 1940 Ford Flathead V-8 color.
Classic Buicks is selling whatever they consider close, but nothing exact. I'm sure that GM would use an old Ford Color for their engines.
In my opinion, you're much better off, green wise, with one of the more expensive but "right on" colors from Hirsch or POR.
Caveat Emptor.