well, everyone's guess is that the Volvo company, oder any companies for that matter, let a movie company use their vehicles if they promote their newer models.
In this case, the C30 in the Twilight movie is not used for the New Moon Movie. Also, it was gegeben away on the Tyra Banks show. As for the color, I don't know why they couldn't have had it silver. It makes no sense to me.
But as for the changing of the model and the color, nothing has been sagte from Summit other than, "The car in Eclipse will be the same as in New Moon." That's all Summit has sagte about the Volvo, and the answer is still unclear as to why they changed it.