How is "naranja(s)" used differently as a noun/adjective in these example phrases?

Question

What is the difference between "las faldas naranja" and "las faldas naranjas?"

Apparently both translate to "the orange skirts," but the first one uses "naranja" as a noun, and the second one uses "naranjas" as an adjective. I'm not sure how the first one is using it as a noun. Is there an error in the book? Is more context needed? Do these phrases actually mean different things?

I found this in the Fluent Forever Awesome word list for (Latin American) Spanish, but it doesn't make sense to me, especially the part "it must remain invariable:"

“Naranja” is mainly the name of a fruit (“the orange”), but it can also be used as the name of the orange color. In that case it can be used either as a noun (in which case it must remain invariable: “las faldas naranja”, “the orange skirts”), or as an adjective (in which case it takes a final -s in plural: “las faldas naranjas”, “the orange skirts”)

Answer 1

The DPD explains this here:

Cuando funcionan como adjetivos, hay que distinguir entre los nombres que designan únicamente colores, los cuales concuerdan siempre con el sustantivo al que modifican (faldas rojas, pantalones verdes, ojos azules, etc.), y los nombres que designan primariamente una flor, un fruto, una sustancia o un objeto que tienen ese color característico, los cuales pueden usarse en aposición y permanecer invariables en plural (ojos malva, faldas naranja, camisas añil, etc.) o concordar con el sustantivo, con funcionamiento plenamente adjetivo (ojos malvas, faldas naranjas, camisas añiles, etc.)

Color names that denote a certain object which characteristically has that color, such as naranja (orange) or violeta (violet) can be placed next to the noun they modify and stay unchanged in plural (this is called an apposition), or they can function as full adjectives and take the plural form if they modify a plural noun.

Color names that only denote colors, such as rojo or verde, must always take the plural form if they modify a plural noun: las faldas rojas.

There is no difference in meaning, and the use of one or other form might depend on the region. In Spain, I have only heard the "full adjective" form: las faldas naranjas.

Answer 2

Here is a simple explanation:

las faldas naranja <- This is wrong, las and faldas both are plural and naranja is singular.

las faldas naranjas <- all are in plural, this is the correct sentence.

I'm from Spain. It's possible that in other countries the Spanish speakers use other words.