[Camillo]

I don't think it makes sense to say that "police" is an adjective in the same sense that "happy" is an adjective. Otherwise all nouns are also adjectives. _Any_ noun in English can be placed in front of another noun to serve as an attribute, because that is a feature of English syntax, but that is different from the distinction between nouns and adjectives as classes of words. The decision to use the word "adjective" both for the part of speech and for the syntactical role is a mistake.