I got to thinking about this when showing some basic Japanese to my parents, explaining how “shiro” means white but you say “shiroi” when using white as an adjective.
If I say “the red book” then obviously red is an adjective. What about “my favorite color is red” or “red evokes passion” or “the wavelength of red is 600 nm?”
Because the first example could be like “my favorite girls are skinny” and skinny isn’t a noun, and maybe you need to say “red light” and not just “red.”
Is there a word for words that can be used as multiple parts of speech?
“My favorite color is red” is much more like “My favorite girl is Deborah” than “My favorite girls are skinny”. In that sentence, “red” is being used as a noun, just as in the following two examples you give.
But, yes, color words can be used as multiple parts of speech. No biggy; many words can (they start as one part of speech (in the case of basic English color terms, I believe historically most started as adjectives (although “orange” started as a noun referring to the fruit)), and then eventually acquire usages in other fashions).
(On edit: There’s no point adopting an analysis under which there is an “implied noun” of “color” in “red evokes passion”; that path leads nowhere useful. Far better to just accept that “red” can itself be employed as a noun and is serving capably in that role in that sentence)
Just to reiterate: in English words can be more than one parts of speech. Nouns can act as adjectives (baseball bat) or turned into verbs.* There is no word for it (well, maybe some obscure technical term) because it is commonplace.
Technically, in the sentence “My favorite color is red,” “red” is a noun. In “My favorite girlfriend is skinny,” “skinny” is an adjective. The verb “to be” has magical grammatical properties that most other verbs do not** however, which means it takes a complement, not a direct object. Thus it can take either an adjective or a noun to finish the sentence (“That water is wet.” “That water is the Pacific Ocean.”)
*Even though verbing weirds language.
**It’s technically called a “copula,” though English teachers use “linking verbs” so the class doesn’t laugh at the name.
Just to nitpick: I am irked by the idea that ‘baseball’ is somehow “acting as an adjective” in ‘baseball bat’. It doesn’t act as an adjective; it just happens to be the case that English allows for noun-noun compounds, despite the misguided elementary school perspective of “Anything that ‘modifies’ a noun is an adjective” or whatnot.
There’s nothing unusual about words taking on new parts of speech as usage evolves, especially with new technology. English speakers won’t hesitate make a verb out of something which beforehand didn’t serve that purpose: “I IMed you last night. Why didn’t you respond?.” Or adjectives to nouns: “I couldn’t take the crazy, so I went home early.” People rarely bat an eyelash at such usage.
In the phrase “the color red”, the word “red” does not serve as an adjective…
You are correct that “red” may be used a noun, with the same meaning as “the color red” or “the color of red things” or whatever. This does not mean it is useful to analyze sentences containing the noun “red” as mere abbreviations for an implicit underlying sentence containing such an expanded phrase, any more than it is useful to consider the sentence “I enjoy rock” as containing an implicit noun “music”, an implicit “listening to” or “the sound of”, an implicit “and roll”, or whatever. Sure, the meaning’s the same, but the syntactic analysis gets confused once you force yourself to invoke phantoms to explain these kinds of constructions, rather than confronting them straight-up.
But whatever. I was only taking issue with the way you presented your point, I suppose, and not so much with the simple point you were making.
I actually don’t even refer to it as a verb when training new teachers, so they get the idea that it’s a different kind of animal entirely. (It doesn’t take do-support, except in the imperative, etc.)
Yes, the terminology changes. However, the OP’s question underscores an important point regarding this. As Indistinguishable says, a word (in English, at least) doesn’t necessary hold some kind of intrinsic character that makes it function as it does. Words acquire their functions through the manner in which people use them. The Wiki article obliquely addresses this:
So you can take a “three-week holiday” or a “three weeks’ holiday.” Who knows, we might see people spending ten dollars bills some day.