"Nesnášejí barvu těchto aut."

Translation: They cannot stand the color of these cars.

[Chessnecof]

When do we need to explicitly write z?

[VladaFu]

When something moves out of somewhere, comes out of somewhere (=from, not of), when you choose out of something and various other cases where the verb or phrase dictates it.

When something just belong the something, that is the most original meaning of the genitive case. Already in Latin and other ancient languages there was no preposition used for that and there is no preposition in the Saxon genitive (car's colour) either.