Gender-Number Agreement with Feminine Adjectives [duplicate]

Question

I take a beginners spanish course in school and came across this blip one day. I was trying to describe a girl in a photo wearing pink socks as la niña lleva calcetines rosas. My teacher then said that I would have to change the rosas to rosa because it didn´t sound natural, then came to the realization that all color adjectives naturally ending in -a stay singular regardless of the noun they are attached to.

We didn't get the time to corroborate the theory with non-color adjective as it was the end of class, but does it apply to all feminine, i.e. -a ending adjectives? Or is the theory correct for all such color adjectives in the first place?

Edit: I believe the question I had unknowingly duplicated had the same essence as mine did, but the answers on the use of the noun/adjective café did not answer my question from the perspective of feminine adjectives, at least from what I understood of it.

Answer 1

It depends a bit on the dialect and is rooted in the type of word that rosa or marrón or carmesí really is.

If a speaker naturally says dos cosas rosa, thev they are actually using a juxtaposed noun. Nouns need not agree with other nouns, especially given that there tends to be an implied phrase that's highly context sensitive and that normally leaves the other noun singular (for example, with colors, dos cosas _{(de color)} rosa).

For many speakers, the terms are/can become lexicalized as adjectives, at which point they also begin to agree in number and gender where morphologically possible and you would get dos cosas rosas.

For an example of this outside of colors, you can look to the term clave. Originally you would see palabras clave, but over time, palabras claves has become more common.

tl;dr it's fine in both singular (juxtaposed noun) and plural (lexicalized as adjective) forms, but dialectal usage may prefer one over another.

Editor's note: "Does it apply to all feminine, i.e. -a ending adjectives?" No. The set of adjectives that are sometimes used without agreement (such as "rosa" and "clave") is small. The standard rule about agreement of number and gender almost always applies, without regard for dialect or region or personal preference.