Don't have the complete details, but they seem to get dirtier when run in racing events. My experience is it takes a couple hundred Kms before they start looking dusty and dull. Darker colored cars seem to show the dirt more easily.
So far my c7 is dusty with 11,000 route x miles , gtr black is pretty dirty but I don't know how I did it and 67 cougar dirty from like 200 rally miles