Just read a small article in the LA Times on car colors in the US for 2012. 22% white; 19% black; 20% silver; 9% red; 7% blue; 2% green. This is from PPG Industries's automotive coating division.
Numbers are actually a bit higher for white and silver in Europe or Asia, according to the article.
One interesting detail: for "luxury cars," gray makes up as much as 33%. Not sure why percentages for gray aren't also included for the general survey. In LA at least grays seem to me almost as common as silvers.