Why is "dark" an adverb in "dark blue"?

The sky is dark blue.

Source:

BBC English

Catherine: The sky is dark blue. The sky is dark blue.

Finn: So, is blue an adjective or adverb?

Catherine: It’s an adjective. Blue is describing the noun sky. Now number two: it's actually the same sentence but this time, think about the word dark. Is dark an adjective or an adverb?

Finn: The sky is dark blue.

Catherine: Right. So, is dark an adjective or adverb?

Finn: Dark here is an adverb.

Thanks for the discussion.

It is interesting to read how the discussion is geared towards the possible explanations of the usage of dark as an adverb.

I've checked several dictionaries before starting this post. Dark is NOT an adverb. If dark is not an adverb, how and why can it be used as an adverb?

In this case, what is wrong to have dark as an adjective in the sentence?

Solution 1:

The answer to the question “Why is dark an adverb in this sentence?” is that it is not one; that source is wrong. That’s because dark cannot ever be an adverb, let alone here. It’s just that color-words can behave somewhat curiously.

We have various related questions about this curiosity, including this one. John Lawler’s suspicion about color words having a special dispensation seems instructive:

John Lawler: Blue is the name of a color; names are usually nouns. I suspect we have a special dispensation for color words, like we do for many other semantically-crucial word sets.

Greg Lee: @JohnLawler, of course "blue" is a noun. I would never deny that, nor does McCawley. That's why, he reasons, it can be modified by the adjective "dark".

So it seems colors can be nouns for modification purposes in that they take adjectives to qualify them. The resulting multiword compound, the adjectivally qualified color like dark red or robin’s egg blue, is used to further describe another noun. The whole compound can then be qualified by intensifiers like very.

_{I'm guessing that we can use adjective ordering rules and perhaps constituency tests to show that the whole multiword part about the color counts as one single syntactic constituent. Therefore you must look at grammatical roles these phrases play in the grammar, not at the internal parts of speech of individual words within that phrase. Otherwise you get nonsense results the way you get when a gerund clause’s head VERB-ing word gets mistakenly called a noun when it’s really a verb. Calling it a noun is a common error — but calling is only a verb there at the start of this sentence, not a noun. I suspect this is the same class of error in calling dark an adverb when it’s actually an adjective.}

What the Dictionary says...

Regarding blue, the OED says:

Often with modifying word indicating intensity (as bright blue, dark blue, light blue, etc.), drawing a comparison with an object or another colour (as indigo blue, lavender blue, powder blue, etc.), or making a (sometimes arbitrary) association with a person or thing (as French blue, royal blue, navy blue), etc.

But those “qualifying words” certainly are not adverbs. Indeed, the OED says that dark is an adjective when it has this color-related sense:

3c. Prefixed, as a qualification, to adjectives of colour: Deep in shade, absorbing more light than it reflects; the opposite of light. (Usually hyphened with the adj. when the latter is used attributively.)

And here are two citations provided of this:

1859 J. Ruskin Two Paths v. 202
That lovely dark purple colour of our Welsh and Highland hills is owing, not to their distance merely, but to their rocks.

1863 M. L. Whately Ragged Life Egypt xvii. 163
Clad in the ordinary dark-blue drapery.

Rubber Baby Buggy Bumpers

Why do they say this is an adjective? If you think it through, you’ll see why dark cannot be an adverb here, only an adjective.

What’s your favorite color?
Hot pink.
You mean like your hot pink sunglasses?
Exactly! All my sunglasses are hot pink!

If someone tells you their favorite color is hot pink, it seems reasonable to call pink a noun and hot an adjective. But now applying that to sunglasses doesn't swap around all the parts of speech into something new.

This happens with colors all the time:

cerulean blue skies
cherry red sunsets
cobalt blue skies
safety green vests
royal purple stoles
saffron yellow robes
robin's egg blue eyes
electric pink sunglasses

The last word in each of those noun phrases is a noun, but the first word of each is not an adverb. It is either a noun or an adjective in each case. Cobalt is a noun; it does not suddenly become an adjective when talking about a cobalt blue nor does it become an adverb when it is used in our cobalt blue skies

This is just like how tinted is not an adverb in tinted glass windows, or like how in the whole tongue-teaser rubber baby buggy bumpers, there are no adverbs nor even any adjectives.

Solution 2:

Adjectives are used to describe nouns, i.e. the car is red.

Adverbs are most commonly used to describe verbs, i.e he fought valiantly

But adverbs can also describe adjectives. How is the car red? Is it blazingly red? Is it cheesily red? Cheekily red maybe?

That is the case for your sentence. The sky is blue. How is it blue? Darkly. It is not being blue lightly, it is being blue darkly.

BUT

It doesn't always need to be this way. One can also have adjective combinations, which would be what most would assume the sentence in question is employing. An adjective combination is where two conceptually separate adjectives join to describe one concept. Dark blue is an adjective, built by two, working in a combination. The sentence in question is ambigious in that sense, as we can't know whether it's an adverb describing an adjective, or an adjective combination. The only person that can really decide that, is the author. But in this case, it doesn't really matter which one it is, as the meaning remains the same either way.

Solution 3:

It's used as part of a set phrase which acts as an adjective. That being said, it's easy to imagine an adverb answering the question "how", as in

Q: "How blue is the sky?"
A: "It's dark blue." <- possibly an adverb, with the meaning that it's mostly black and a little bit blue

A: "It's slightly blue" <- definitely an adverb