Mercury is red.
Mercury is the color red.
Red is describing Mercury. What part of speech would color be?
closed as not a real question by kiamlaluno, Matt E. Эллен♦, RegDwigнt♦ Dec 19 '11 at 13:19
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It's an odd sentence, which may not be strictly grammatical. However I would probably not find it jarring if I heard or read it, so it may be a common idiom.
"noun is noun" sentences are normally denote equality or set membership. "Obama is the president", "Rover is a dog", for example.
By that logic, the sentence is not true. "Mercury" is not "the colour red".
"Mercury is red" is a "noun is adjective" sentence. "David is hungry", "Madelyn is pretty".
The "the colour" part of the sentence could be seen as a clarifying clause: let us imagine that the adjective "red" is ambiguous. Perhaps the reader might think it means "communist". Perhaps we are speaking aloud and the listener might hear "Mercury is read".
So, we clarify by adding a clause saying "the colour".
Another way to look at this is to view "the colour red" as an adjectival phrase.
In "John is fond of animals", "fond of animals" is an adjectival phrase. "Fond" is the adjective and "of animals" is a clarification of the adjective.
In "Mercury is the colour red", "the colour red" is an adjectival phrase. "Red" is the adjective and "the colour" is a clarification of the adjective.
In all these forms "colour" is a noun, even though red is an adjective. The final form may be ungrammatical, but likely to be found acceptable in all but the most formal settings.
The answer depends on which color you are referring to. The word color is used in the different sentences as different parts of speech.
The word color can be used both as a noun as well as an adjective.
'Mercury is the color red' should properly be one of the following:
Red can be both an adjective and a noun. All color words in English function this way.
As an adjective:
As a noun:
Mercury is the colour red asserts that the two concepts are the same, which is clearly wrong, and only just avoids being nonsense. Mercury is coloured red may be what you are looking for, in which coloured is part of an adjectival phrase.