If you're willing to trade, you can easily get the missing colors.
Probably the most efficient way is the birthday glitch, but that could maybe get you banned from the PSN for false account info. You can solve that by getting the appropriate bday ticket from someone who already took that risk, then duping it yourself until one gives you the car and color you seek.
The "completely legal" way of doing it is to wait for the car in question to show up in the Online Collector's Dealership. I heard everyone sees different colors in there. If that's true, then someone, hopefully someone on GTP, will probably have the car you want in the color you want, and hopefully they'll be willing to trade it for something (if you're lucky, it'll show up for you in that color). IF you manage to do all of that, you've got the car and color you were looking for and no one had to risk bans to get it for you.
