Colors are adjectives! Colors describe what the noun is like, so they must agree in number and gender with the nouns to which they are attached. Colors that end in vowels will end in 'a' to agree with feminine nouns or 'o' to agree with masculine nouns. If a color ends in 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 gato negro - the black cat
los gatos negros - the black cats
la pared azul - the blue wall
las paredes azules - the blue walls

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.