In similar exercises, others have explained that "can't stand" (nesnášet) and "hate" (nenávidět) vary in degree, with the latter being stronger. So "hate" may not be accepted in the translation, since it was not used in the original.
Also, but probably less important, "these cars' color" just sounds strange and awkward, although it's not grammatically incorrect.
Actually, this exercise does accept hate but does not accept "these car's color" because it sounds strange. But if it is grammatically correct it probably could be accepted.
There is also the question of whether we have one color (of all the cars) or several colors (one for each car). My Czech is not good enough to tell whether barvu is singular or plural. That could move the apostrophe from color's to colors'.