Certainly being visible at a distance would seem to be a good thing (I trust in the smart's crash worthiness, but I'd still rather not test it in person).
I think that the key to visibility is the difference in contrast between the color of the car and the color of the background (i.e.
the color of pavement, which is typically somewhere between light and dark gray). In which case, highly saturated colors like yellow and red would be best, followed by blue, then white, black and silver. I think that silver, which is essentially metallic gray, would offer poor visibility, at least in normal daytime conditions.
On a non-safety note, black and white are a pain to keep looking clean. But that's just me, because I'm lazy about washing my car. Silver, on the other hand can go for years without needing cleaned.