The colors shown in the rewards section are the actual production colors. The idea is that the singles have different colors than the 3 pack. This way every time you add a car...even if it has the same body style, you increase the number of cars you can make by swapping cars EXPONENTIALLY!!!! If you sign up for the company picnic you get all the combinations.
As if that's not enough options...get the DIY and paint your own colors.