If I had been posed this question I could imagine it having gone two different ways. First, I could approach it as remembering, maybe not a car I saw yesterday, but a generic idea of some type of car. For example if I thought about a Japanese pickup or a Lamborghini, I would probably answer the following color question with "white" and "orange", because for me those are stereotypic colors that "come with" the memory. Perhaps many people think in this way and thus assume the color obviously is there.

The second way would be to abstractly build a car in my mind. Start with four tires, put a rough shape of a frame on it — lets make it a sedan. If I proceed this way, I probably would not choose a color, at least not early on. Like many in this thread, this is how I usually imagine things, as "wireframes". The color/texture is not there because I have not assigned it. I can't vividly see the things I imagine, but they can still have color just like they can have shape.