We have been playing different variations of this game for years.
Here is how you play Yellow Car: Everyone looks for yellow cars. The game scores as follows: regular yellow car - 1 point, any yellow pick up truck - 5 points, yellow Mustang - 5 points and yellow Hummer - automatic win! We keep score until someone finds a yellow Hummer but you could just do it for a certain amount of time. We also play this game over multiple car rides. We just remember our score and pick up where we left off.
Then over time we mixed it up and play a cooperative version called Rainbow Car. Here is how you play rainbow car: Work together to find cars in the color of the rainbow. You must do it in order. Therefore, first you look for a red car, then orange, yellow, green, blue and finally purple. When you are done you start back over at red again.
Happy car hunting!