One thing about the car color which I may have missed but I haven't read/heard--dark(er)/night lighting comes into play, and a car of a certain color can be mistaken for another shade altogether.
Very true.
I also noticed that the top of Kim's car has light gray, peeling paint that looks almost white. If she has a lot of that on the hood as well, the cabbie could have easily remembered it generally as white. It probably was of no importance to him at the time, and not the sort of thing one would remember well - not like one would remember who they picked up, what was said, where they went, and so on.