I've never seen an 'official' sales breakdown by color. Even the WRX vs. RS sales aren't separated in the general reports. Not saying it doesn't exist...just that it may not make its way outside of corporate.
Given that Rally Blue has the ties to Subaru's WRC car, I'd be highly suprised if any of the other colors sold in higher quantities. FWIW, across all manufacturers ,silver is far and away the most popular color choice.
Cheers,
Ryan Douthit
SUBIESPORT Magazine
http://www.subiesport.com