The green Scamp is feeling blue...

I guess it mostly depends on what kind of paint is on the car, and what kind of paint the spray paint was. Different solvents will harm, or not harm, the car's finish. But I think the safest way to remove the paint would be to buff it off with some fine compound. Not sure if the "clay" mentioned above will be aggressive enough, but maybe. That paint really ought to come off easily, since it probably hasn't totally cured yet.