I think the following two sentences have the same meaning :
- Your shirt is of the same colour as John's.
In this sentence, the PP ("of the same colour") modifies the NP ("Your shirt"). Here the PP is adjectival in function.
- Your shirt is the same colour as John's.
In this sentence, the NP ("the same colour") modifies the NP ("Your shirt").
Thus, can the NP ("the same colour") be adjectival in function? Could you please give similar examples of the NPs, being used as an adjective?
I prefer to call it attributive rather than adjectival, but I must agree that nouns and their phrases can have such a function.
Phrases like "a toy box", "a model airplane" and "a windshield wiper" use common nouns to indicate the attributes of other nouns. "Toy" represents the purpose of the box, or the kind of thing it is intended to contain. "Model" indicates things like the nature, size or utility of the airplane. "Windshield" represents the thing that is wiped.
A shirt the same color as John's is hanging in that window.
We can paraphrase the sentence above with "a shirt of the same color..." or "a shirt which is the same color..." or "a shirt that has the same color..." -- constructions which have prepositions or verbs governing the noun phrase in question. As it appears above, there isn't anything governing that association. At least, there isn't anything visible. We would have to posit things like null prepositions or whiz deletion, if we're going to claim that the attribution is governed by something which takes it as an argument.
The obvious alternative to positing invisible governors is to assume that this noun phrase is an unlicensed attribution, much the same as the "toy" of "a toy box".
Your shirt is red.
Your shirt is the same color as John's.
Your shirt is a part of a Star Trek uniform.
The first sentence in this group has a predicate adjective subject complement. The third sentence has a predicate nominative subject complement. Your question is whether the subject complement of the middle sentence more closely resembles the one above it or below it.
The nominative complement has the same referent as the subject. "Your shirt" and "a part of a Star Trek uniform" mean different things, but they both indicate the same garment in this context. The adjective complement does no such thing. You and I might agree that it is a modifier, although I would call it an indirect or licensed modifier, since it is a constituent of the predicate and an argument of the verb. Someone else might reserve the word modifier to mean only direct modifiers, and refer to licensed modifiers only as a type of complement. Either way, the noun phrase doesn't fill the same grammatical role as the red of "your red shirt", despite carrying much the same semantics.
If we must label "the same color..." as either adjectival or nominative, then it's adjectival. Chances seem good that your preferred grammar books use exactly that phrasing. The ideas of this shirt and this color don't represent the same referent, not even when we're pointing at the same garment. It has a referent all its own, indicating one attribute of the subject.
Since it consists of a noun with its modifiers, rather than an adjective with its modifiers, I'll call it a predicate attributive subject complement. It's doing the same job that an adjective in the same position does. It does that job in its capacity as an attributive noun phrase.
[1] Your shirt is of the same colour as John's.
[2] Your shirt is the same colour as John's.
Yes, [1] and [2] have the same meaning, but NPs cannot be adjectival, as I explained to you in comments.
In [1] the NP "the same colour" is complement of the prep "of", and the whole PP "of the same colour ..." is complement of "be".
In [2] the NP "the same colour ..." is subjective predicative complement of "be".
The crucial points are that predicatives may be AdjPs or NPs, and they are not modifiers of the subject or object, but complements of the verb:
[1] The house was white. [AdjP as subjective PC of "be"]
[2] We painted the house white. [AdjP As objective PC of "paint"]
[3] Ed was a teacher. [NP As subjective PC of "be"]
[4] They elected him treasurer. [NP as objective PC of "elect"]
We refer to the PCs in [1] and [3] as subjective (or subject-oriented), and those in [2] and [4] as objective (or object-oriented). Importantly, in all cases the PC is complement of the verb, not the subject or object.
Note that it is important not to conflate the terms word class and function. NPs and AdjPs are the corresponding phrasal categories of the word classes adjectives and nouns. By contrast, complement and modifier are functions.
EDIT: I don't use the terms 'predicate adjective' and 'predicate noun', as it's obvious whether a PC is an adjective or a noun. But for those who do use those terms, where the PC is an adjective it can be called a predicate adjective (or AdjP) and where the PC is a noun (or NP) it can be called a predicate nominative:
[5] Ed seems nice. [predicate adjective subj complement]
[6] Ed is a teacher. [predicate nominative subj complement]
Importantly, although the adjective "nice" and the noun phrase "a teacher" both refer to the subject, their function is complement of "seem" / "be".
