Colors (los colores) are adjectives — they must agree in gender and number with the nouns they describe.

The Basic Colors

• rojo/a — red
• azul — blue (same m/f)
• verde — green (same m/f)
• amarillo/a — yellow
• naranja — orange (invariable)
• blanco/a — white
• negro/a — black
• gris — gray (same m/f)
• marrón / café — brown
• morado/a — purple
• rosado/a / rosa — pink

Agreement in Action

• El carro rojo — the red car
• La camisa roja — the red shirt
• Los zapatos negros — the black shoes
• Las flores amarillas — the yellow flowers

Invariable Colors

Colors borrowed from nouns (naranja, rosa, violeta) do not change form: una blusa rosa / unos zapatos rosa.

Reference:

Wikipedia: Spanish Vocabulary

https://en.wikipedia.org/wiki/Spanish_language