Only when the adjective is the predicate of the sentence does it match its noun in number. Like in the sentence above: "autók" is the subject, "pirosak" is the predicate. In such cases it's not possible for the noun to be in any case other than nominative.
When the adjective is attributive (so it's directly before the noun), it always stays in its base form:
- "a piros autók" -- "the red cars"
However, when an adjective substitutes a noun, it takes every suffix the noun would take:
- "Látom a piros autókat." -- "I see the red cars." -> "Látom a pirosakat." -- "I see the red ones."
I believe that when the adjective comes before the noun, it is singular irrespective of the number of objects, just as in English - the old car = a régi auto, and the old cars = a régi autók.
However when the adjective comes after the noun, forming a complete sentence, it is treated the same way as you would a noun in the same position: eg the cars are vehicles = a régi járművek, and similarly the cars are red = az autók régiek. In this instance régi is indeed plural.