[insert a bang and a cloud of smoke *here*]
>>>I don`t know if you can sue them because they are only accepting silver cars but it should be possible under German law because owner of other colored cars can feel discriminated.
No, absolutely not. This is a private business, they´re not obliged to treat everybody equally.
If this was an airline belonging to the state there might be a point of discrimination. However, the state may discriminate if there´s a good reason to it, and I´d say this point of "silver plane/silver car/good brand recognition" is a good one.
Technically, this campaign is a public prize offering with a limited eligibility. Or in German Legalese an "Auslobung ( = einseitige nicht empfangsbeduerftige Willenserklaerung)".
I´m not sure whether this campaign might violate the Law Against Indecent Or Unfair Competition (Gesetz gegen unlauteren Wettbewerb). I don´t think so, though. In any case, that law protects the competitors, not the customer, and consequently no individual´s claims can be based upon it.
So, sorry Patrick, no damages for owners of blue cars