Los colores

Colours arrive slowly in a frequency course, because people say white and black constantly and purple almost never. Blanco and negro are early words. Morado is not.

Now they are all in one place, and the grammar is mostly easy with one sharp exception.

Most of them agree

Un coche rojo, una casa roja, unos coches rojos. Colours are adjectives and they behave like adjectives: they follow the noun and they match it in gender and number.

The ones ending in -e or a consonant only change for number: verde, verdes. Gris, grises. Azul, azules.

The ones that do not change at all

Naranja and rosa are fruit and flower first, colour second. When they are used as colours they are really shorthand for the colour of an orange, and a noun cannot agree with anything.

So traditionally unos zapatos naranja, not naranjas. Unas cortinas rosa, not rosas.

You will hear both. Rosas and naranjas as plural adjectives are common in speech, and the RAE now accepts them, but careful writing still tends to avoid them, which is exactly the situation English is in with less and fewer.

Describing people

Spanish uses a separate set of words for hair and skin, and using the object colours instead sounds wrong.

Moreno is dark-haired or dark-skinned. Rubio is blond. Castaño is chestnut brown, for hair. Pálido is pale.

You do not say someone has pelo amarillo. You say rubio.

Moreno is worth a note: across most of the Spanish-speaking world it is neutral and descriptive, applied to hair, tans and complexions alike, though in some countries it is also used, often euphemistically, for a Black person. It is not the loaded word an English speaker might assume from its shape.

claro and oscuro

Claro is light, oscuro is dark, and both go after the colour without agreeing: azul claro, verde oscuro. In careful Spanish the whole pair then stays put: unas camisas azul claro, ojos verde oscuro.

Claro on its own is also of course, and it is one of the most common conversational words in the language.