The liquid in the bowl turned a bright blue.
Please tell me if blue is a predicate nominative or a predicate adjective in this sentence and please explain why.
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I would say the article and the adjective bright force us to interpret blue as a noun here, so a bright blue becomes a noun group.
The liquid's color is what changed to blue; the condensed matter of the liquid did not literally change into photons in the blue region of the visible spectrum. Assuming the lack of exactitude to be intentional, the original sentence adequately conveys an observer's feeling of the importance of the eye-catching blueness resulting from some transformative process. For the sentence author, and reader, the fact that "blue" is a noun should be counted unimportant.
With that in mind, consider the identically constructed sentence "The grammar girl turned a beet red." This inexact sentence focuses the reader's attention to the uncomfortable changes in the color of the girl's face. Although "red" is a noun and in the predicate, it is not functioning as a predicate noun for girl; "red" is not renaming "girl".
In the original sentence, the noun "blue" is not renaming "liquid": it is renaming the color of the liquid.
'To turn ' is a linking verb (copula) and is followed by either predicate nominative or predicate adjective better known as complements. Predicate nominative is , in a way, renaming of the subject and predicate adjective is describing it.
In our instant example, 'a bright blue ' is describing the liquid. 'Blue' is an adjective modified by 'bright' an adverb; but preceded by determiner 'a', an article, the phrase, 'a bright' functions more like a degree adverb.
The Cambridge Dictionary gives us certain illustrations of words and phrases that go before and after adjective. They are adverb phrases of degree. An example:
In the example sentence , "a deep" together with 'blue' functions in the like manner in describing the subject.