I hadn't seen the 60% number and went looking. If this source is the one you were referring to, 60% refers to the percentage of sunlight reflected by a light colored car. A black car reflects about 5%. The study showed the interior temperature of the black car was 5-6 degree Celsius higher than that of the silver car. The authors also concluded you can boost your gas mileage by 2% by driving a light colored car, assuming the air conditioning is running.
Cool Colors for Cars Could Improve Fuel Economy, Reduce Emissions