Using Adjectives as Nouns in Spanish
Adjectives are often used as nouns to refer to a noun that was recently mentioned.
Forming Nouns with Adjectives
To use an adjective as a noun, you simply add an article. The article must match the noun it refers to in gender and number.
Since the noun vestido is masculine and singular, rojo is also masculine and singular.
Remember that when a person is the direct object of a verb, you must use the personal a. When a precedes el, they combine to form the contraction al. This same contraction rule applies to de + el, which together form del.
Use with Demonstrative Adjectives
Demonstrative adjectives may also used with this type of noun.