I find it quite difficult to assess just how popular the colour was as the production run of 313's was so limited. For sure, Rosso Red, Pearl White, Temper Orange and Midnight Blue Zeds are a rare sight on our roads. The other 3 colours available were either identical in the case of black or almost identical to the previous model colours, ie Universal Silver/Blade Silver and Twilight Grey/GM Grey. As an ardent spotter of Zeds I've actually seen more Midnight Blues (3) than Universal Silvers (2)
Unless you get a good view of the front bumper/lights they are really difficult to tell apart and of all the Zed colours Silver shows off the bulge bonnet the least.
Also, the dynamics of Zed sales changed completely in 2007, previous to that it was pretty much impossible to buy a new one off the shelf. The 3 main dealers I went to in 2006 were all singing from the same hymn sheet. Chose your colour/spec, give us 2 grand and we'll phone you in 3-4 months when it comes in . After the bankers crises in 2007 there were brand new Zeds to be had almost on demand, of course, you could still order one to your exact spec, when I enquired for Caroline in early 2008 I was told "a few weeks delivery" but as long as I wasn't too fussed about colour/spec then they could get me one within a week
Pete