Most people don't change their colours. Or they pick from one of the standard sets and then go back to the default some days later. I know this because I *do* change my colours. Not only do I get incredulous comments from colleauges, but I am constantly finding applications that - often unconsiously - assume the end-user hasn't changed the system colours.
On the other hand, it is hard to find someone who *hasn't* change their wallpaper. :-)
Wade.