My DH's policy has always been to buy a new car, because it will be the most up to date and energy efficient. We then keep them till they give up on us, or, as in the present case, until the (German) government offered a generous grant for scrapping old, unecological cars and buying new state of the art ones.
Our previous car was on the way out anyway and we were downsizing because the kids had all left home.
As I said before, however, whether the car looked clean or not was never a criterion.
I would strongly advise against silver cars. For most of the year, they are the same colour as the road and the sky and are practically invisible.