Hazel Eyes

I found the following paragraph in the guycounseling.com blog article “Hazel Eyes: Learn Why People with Greenish Eye Color are Rare!”, containing the two words “hazel eyes”:

Hazel eyes are fascinating to gaze into. When you look at someone who has hazel eyes, you see colors that are completely different than other eye colors, such as crystal blue or emerald green.

—Guy Counseling Site

I also found some text on another site that says:

Her eyes are hazel.

Hazel is a noun that denotes a colour; how can hazel modify the noun eyes if hazel is also a noun itself? Doesn’t a word always have to be an adjective to modify a noun? Isn’t that what “adjective” means by its very definition: a word that modifies a noun? How can something modify a noun without being an adjective? Is that even possible?

The dictionary entry for hazel I found on the online Oxford Living Dictionaries website doesn’t mention that hazel can ever be adjective; it mentions only that it is a noun:

hazel

ɴᴏᴜɴ

1. A temperate shrub or small tree with broad leaves, bearing prominent male catkins in spring and round hard-shelled edible nuts in autumn.

   ^{Genus Corylus, family Betulaceae: several species, in particular the common Eurasian hazel (C. avellana)}

2. [mass noun] A reddish-brown or greenish-brown colour, especially of a person’s eyes.

   [as modifier] ‘the laughing hazel eyes were serious now’

—Oxford Living Dictionaries

How can this be grammatical?

Her eyes (noun) are hazel (noun)?

And also, if you accept that a noun can work as an adjective, then, can adjective work as a noun?

Two different facts are needed to thoroughly answer this question.

First, Hazel is both an adjective and a noun. Online dictionaries can be hit-and-miss in quality. Even high-quality dictionaries make mistakes, or will be abridged, and different lexicographers have to pick and choose what to leave out. When researching a topic like this, you owe it to yourself to check more than one dictionary. (Maybe even invest in a high-quality PRINT dictionary.) For example, Merriam-Webster online shows Hazel is both an adjective and a noun, but you have to scroll the page about a third of the way down to find the adjective form: https://www.merriam-webster.com/dictionary/hazel

And the American Heritage online dictionary also lists it as both a noun and an adjective: https://ahdictionary.com/word/search.html?q=hazel

Dictionary.com also shows it as both a noun as an adjective, but you need to scroll to see the adjective listing: http://www.dictionary.com/browse/hazel?s=t

By the way, Oxford Dictionaries seems inconsistent in their parts-of-speech classification of colors. Red and green are listed as both nouns and adjectives. If you look up amber, it's listed only as a noun, but one of the example sentences actually uses it as a modifier for eye color:

1.1 A honey-yellow colour typical of amber.

• 'her eyes were green flecked with amber'
• [as modifier] 'amber eyes'

Second, parts-of-speech in English aren't as clear-cut as they are sometimes taught. The part-of-speech of a word often depends on how it is used. It's very common for a noun to act as an adjective. (In fact, it happens so often, that I'm surprised this hazel eyes example might be the first time it's come to your attention.) Examples are a "pet store," a "shoe factory," a "couch cushion," or a "cucumber sandwich." Nouns that act like adjectives are called "attributive nouns," and this page gives an overview:

https://learningenglish.voanews.com/a/everyday-grammar-when-nouns-act-like-adjectives/2998821.html

Incidentally, verbs can also modify nouns, the so-called "attributive verbs": https://en.wikipedia.org/wiki/Attributive_verb

So either way, whether you accept hazel as an actual adjective, or a noun acting like an adjective, the expression "hazel eyes," is grammatical.

Can an adjective act as a noun? Colloquially, yes. If offered hot or cold tea, I might answer, "I'll have the hot." Or a customer in a hardware store may ask an attendant: "Where is your electrical?" (Meaning where is your electrical section.) I'd be wary of using similar constructions in formal speech or writing. You can see several other examples on this StackExchange page: Is there a term for the use of adjectives as nouns?

In a comment, BillJ wrote:

I'd take "hazel" to be an adjective, so in the NP "hazel eyes", "hazel" is an adjective modifying "eyes". And in "Her eyes are hazel", "hazel" is an adjective as predicative complement.

In a comment, NigelJ wrote:

'Hazel' is only a noun when it applies to 'a type of deciduous shrub or tree' or when it applies to the actual colour called 'hazel'. Otherwise it is an adjective describing something that is 'of the colour, hazel' OED.

Hazel in your example is a colour name, and hence functioning as an adjective. Some colour names are adjectives in their own right (e.g. red, brown, yellow, etc), and many others are derived from nouns denoting a coloured thing. An orange is a fruit; indigo is a plant from which a dye is made, a violet is a flower; hazel is a plant with greenish-brown parts. Some shades of green are described as 'avocado'; some greys are 'charcoal'.