I'm trying to figure out what colors the various generations were painted by the factory...
Were there special edition colors?

I saw a gold/orange car for sale and I was wondering if it was a custom.

Anyway, if you know the answer or of a NSX specific site with the info LMK. :p

You probably saw Imola Orange Pearl.

http://www.nsxprime.com/FAQ/General/changesbyyear.htm

I found that site a little after I posted...
The car in question is a '91 with a color keyed interior, so, a factory style custom...?
:p

Sounds like it. Maybe the owner converted it to late-model parts.