Adjective preceding attributive nouns

When an adjective is preceding two nouns, the first one being an attributive noun, does it define the final noun or the attributive noun?

For example: Red car keys

Are they red keys that open a car, or are they keys that open a red car?

Answer

Strictly speaking, the meaning is ambiguous in your example phrase.

It's more common in this case to think of the first noun as modifying everything that follows (red keys that open a car), and I doubt that somebody would actually interpret it the other way (keys that open a red car).

So, at least here, I don't think ambiguity would be a practical concern. But it might be with other constructions.

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Context can determine meaning:

Jimmy's only car was yellow. He liked to put his red car keys in a cookie jar on his kitchen counter.

Here, the surrounding text makes it clear that it's Jimmy's car keys that are red and not his car.

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Hyphenation can also be used to avoid ambiguity if the meaning is not made obvious from context:

Jimmy grabbed his red car-keys.
Jimmy grabbed his red-car keys.

At least in this case, however, I would say that even though the meaning is now clear, the sentences look strange. It's not normal to see either car-keys or red-car in this kind of construction.

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If context doesn't clear up the ambiguity, and hyphenation isn't a good style choice, then the only other option is to rephrase:

Jimmy grabbed the red keys to his car from the kitchen counter.
Jimmy grabbed the keys to his red car from the kitchen counter.