According to Renntech, the code is 3AU (water based paint). It's 3AT for the conventional paint. When I got my side skirts painted this yr, I had the paint shop use the water based paint code for Arctic Silver, and it came out matched perfectly. Oh, I gave them one of my painted bumperettes to help them match too.
http://www.renntech.org/forums/index.php?autocom=content&code=colors_boxster