I could imagine everyone re-covering their saddles white to go with their posh fixies… again, all food for thought.
You got it! White is the most popular fixed gear colour, but those crazy kids quite like a bit of neon as well (doubt horses feel the same way) and, in general, the more colours the better so that the bike can be fully colour coordinated. Think all of the colours that have ever been, or could ever be, on a My Little Pony and you are starting in the right place. The personalisation would be good too as saddle theft is a problem here in London – in fact there's already a guy who stamps initials etc on brooks saddles.