Originally Posted by L39E

Riviera Blue was a standard color only in 1995. I believe there were 7 cabs and 17 coupes made for the USA. After that it was PTS. Blue Turquoise was a standard color in 1996 and 1997. Polar Silver was standard in 1995 and 1996, Arctic Silver in 1997 and 1998. I know there is a chart with the standard colors for the respective model years. I will try to find it. EDIT: here is a document from Porsche. No info for 1998, maybe Euro model years as it has 1994.

993 Carrera - Overview Paint Codes