Color adjectives agree with the nouns they follow.
Un oiseau bleu. Des oiseaux bleus. Une fleur bleue. Des fleurs bleues. A blue bird. Blue birds. A blue flower. Blue flowers.
Color adjectives derived from nouns such as animals, flowers, and fruits, are usually invariable. Examples: orange (orange), marron (brown/chestnut), cerise (cherry), crème (cream), pastel (pastel), turquoise (turquoise), bronze (bronze), etc.
Des pantalons marron (not
marrons). Brown trousers.
Color adjectives that are modified by another adjective or noun are invariable. Examples: bleu marine (navy blue), vert clair (light green), jaune pâle (pale/light yellow), etc.
Des chemises vert clair (not
vertes claires) Light green shirts.
• Rose (pink), mauve (mauve, lilac) and pourpre (purple) are exceptions. Although they are derived from nouns (mauve is a type of flower and pourpre is a snail), they take an -s in the plural form.
Des pulls roses Pink sweaters.
• Two color adjectives are joined by a hyphen.
Des yeux bleu-vert, des yeux gris-bleu Blue-green eyes, grey-blue eyes.