|
As Robindesbois indicates, some color names are fundamentally adjectives, and some of them are fundamentally nouns.
When a color name is fundamentally an adjective, the adjective has different forms that agree in number with the noun they describe (i.e., verde/verdes, azul/azules), and some of them also have different forms that agree in gender with the noun that they describe (i.e., rojo/roja/rojos/rojas, blanco/blanca/blancos/blancas).
When a color name is fundamentally a noun, it usually is invariable in form when used adjectivally: that is, it has only one form regardless of the gender and number of the noun it describes. The underlying structure is the phrase "(de color <color name>", (i.e., de color rosa, de color violeta, de color turquesa). In general, it is optional to use the complete phrase "de color <color name>" or to omit "de color".
The structure "de color <color name>" is also perfectly fine for color names that are adjectives: "de color amarillo" and "de color negro" are perfectly fine, and in this case the adjectives are always masculine singular because grammatically they are describing the word "color".
Sometimes as Spanish changes over time a color name that was original fundamentally a noun can acquire the status of being an adjective; that can happen if native speakers generally consider it correct to use different forms for number agreement or for both number and gender agreement.
There's no absolutely fool-proof rule to know instantly whether a particular color name is invariable or has forms for number or gender agreement: we learners have to learn it individually for each color name.
|