Just a little bit of info on this, not really a solution, but this didn't unlock for me until i had 261 paint colours, so if it doesn't unlock at 256 just keep getting cars and it should eventually unlock.
Also if you're a newcomer to GT5 like i was, got PS3 and GT5 20 days before server shutdown, this will be a lot more of a grind as you use to get daily paint colours for logging in, but now you will have to get at least 256 cars with different paint jobs to unlock this.