Customize "Non-Customizable" Vehicles
Normally when you attempt to take these "special" vehicles to a Rim Jobs Garage it'll tell you they're non-customizable, but here's a way around that.
First, find any non-customizable vehicle and save it in your garage. This will work with the Prof. Genki and Nyteblade DLC vehicles too.
Go to any Rim Jobs Garage with a customizable vehicle, then go to your garage from the Rim Jobs menu and highlight a non-customizable vehicle in your garage, then press B to go back and select Accept. You should now be in the garage with the so called non-customizable vehicle. From there you can change the colors if you wish, body mods are usually pretty limited since we're not supposed to be able to edit them ourselves.