Simple colour adjectives (bleu, vert, rouge, and so on) agree normally. Colours derived from a noun (orange, marron, crème, and so on) and compound colours (bleu foncé, vert clair, and so on) stay unchanged.
Simple colour: make it agree. Colour from a noun (orange, marron) or a compound colour (bleu clair): unchanged.
Examples
Des yeux bleus.
Key word(s): bleus
A simple colour, so it agrees normally.
Des chaussures marron.
Key word(s): marron
Derived from the noun "marron" (the nut), so it stays unchanged.
Des robes bleu clair.
Key word(s): bleu clair
A compound colour, so it stays unchanged.
Des chemises vertes.
Key word(s): vertes
A simple colour, so it agrees normally.
Des murs orange.
Key word(s): orange
Derived from the noun "orange" (the fruit), so it stays unchanged.
Common mistakes
Making "marron" or "orange" agree like ordinary adjectives
Making compound colours agree ("des yeux bleus clairs")
Exceptions: rose, mauve and pourpre do agree despite their origin
Test yourself
Des fleurs ___.
"Rose" is an exception, so it agrees.
Des gants ___.
"Orange" comes from the name of the fruit, so it stays unchanged.
Des yeux ___.
A compound colour, so it stays unchanged.
Tips to remember
Colours that stay unchanged (nouns): orange, marron, crème, cerise, turquoise, kaki, and others.
Exceptions that do agree: rose, mauve, pourpre, écarlate.
As soon as there are two words (bleu clair, vert pomme), the colour stays unchanged.
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