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 end in 'a' to agree with feminine nouns or 'o' to agree with masculine nouns. If they end with e it remains the same. If they end with a consonant, such as 'azul', it doesn't change. To pluralize a color that ends in a vowel, simply add an 's'. If the color ends in a consonant, add 'es'.
For example:

El perro es negro. - The dog is black.
Tú pediste zapatos negros - You asked for black shoes.
Yo quiero la camisa azul. - I want the blue shirt.
María tenía los ojos azules. - María had blue eyes.

Let's look at some example sentences:

Está en la caja café. - It is in the brown box.
Yo prefiero el arroz blanco. - I prefer white rice.
Ella tiene un vestido violeta. - She has a violet dress.