Colors are adjectives! They describe nouns, so they usually agree in number and gender with this nouns. Colors that end with vowels a, o will often end in 'a' to agree with feminine nouns or 'o' to agree with masculine nouns.

Yo prefiero el arroz blanco. - I prefer white rice.
El perro es negro. - The dog is black.

If the color ends with e it remains the same.

Está en la caja café. - It is in the brown box.

If the color ends with a consonant', it doesn't change.

Yo quiero la camisa azul. - I want the blue shirt.

To pluralize a color that ends in a vowel, simply add an 's'.

Tú pediste zapatos negros - You asked for black shoes.

If the color ends in a consonant, add 'es'.

María tenía los ojos azules. - María had blue eyes.