Isn't this a perfect case for using the CoronaMultiMap?
Let's say you have 1 car model which consists of various parts. You assign various material IDs to those parts. Some of them are using a carpaint material, some of them are plastic, others leather, etc.
You assign a multi/sub-object material to this car and make sure that each part has a correct Corona Mtl applied to it.
Then you plug the CoronaMultiMap into the CoronaMtls' diffuse, reflectivity, or whatever you need.
You set the MultiMap's mode to per-instance randomization.
You clone the car 100 times.
You end up with 100 cars using various combinations of carpaint, plastic, leather, and other colors take from the multimap.
For additional control you may change MultiMap mode for example to object ID, so that you can decide which specific car should have which color scheme.
Does it make sense, or do you need something completely different?