This slideshow walks through issues around when to modify colors to match the gender of the object being described.
Outstanda : Spanish Colors and Shapes
Color can be adjectives. They must agree in gender and number with the noun they modify. Any color adjective in Spanish that ends with -o is meant to change to agree with the gender of the noun it is describing.
Amarillo: El libro es amarillo. La pluma es amarilla.
Blanco: El libro es blanco. La pluma es blanca.
Rojo: El libro es rojo. La pluma es roja.
Notice how the -o changes to an -a in the feminine form.
Compound colors like Navy Blue do not change gender.
Azul Marino (Navy Blue): El libro es azul marino. La pluma es azul marino.
Verde Oliva (Olive Green): El libro es verde oliva. La pluma es verde oliva.
Colors also become plural when they refer to multiple items.
One red apple = Una manzana roja.
Two red apples = Dos manzanas rojas.
Un libro azul. Tres Libros azules.
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