From Wikibooks

The rules for forming the feminine of adjectives are different from those for the noun

Regular inflection of the feminine.

If a word ends in a vowel, the feminine replaces that vowel by a: el libro rojo, la casa roja. If a word ends in a consonant, the feminine adds an a: el libro inglés, la casa inglesa.

When adjectives are inflected

Adjectives are inflected or not depending on the ending of the masculine-gender form.

• o: inflected. el libro nuevo/la casa nueva. Exception: el departamento piloto/la casa piloto.
• a: not inflected. el amigo barsa/la amiga barsa. (Barsa is a slang term; a better example is sought.)
• (*)e: never inflected. el libro verde/la casa verde.
• ante: not inflected. el piojo cargante/la mosca cargante.
• (*)ente: never inflected. el alumno ausente/la alumna ausente.
• Stressed vowel: not inflected. El libro café/la casa café.
• (*)Consonant: not inflected, but see regular exceptions below. El libro azul/la casa azul.
• ín, ón: inflected. El camello bailarín/la jirafa bailarina. (*) Exception: el libro marrón/la casa marrón.
• or: inflected. el cable conector/la cuerda conectora. (*) Exceptions: comparatives such as mayor, menor, mejor, peor, anterior, poterior, superior, inferior.

Adjectives of nationality ending in és are inflected: el libro portugués/la casa portuguesa.

(*) indicates that the rule is different for adjectives and for nouns. For example: el superior/la superiora, but el libro superior/la casa superior.