In French, adjectives normally have to agree with the nouns they modify in gender and number. However, there are numerous adjectives which don't agree - they have a single form that does not change to reflect the gender or number of the noun. These are called invariable adjectives.

French color adjectives derived from nouns, such as animals, flowers, fruits, gems, and metals, are usually invariable....

Exceptions:

1) alezan (chestnut), fauve (tawny/fawn), incarnat (rosy), mauve, pourpre (crimson), rose (pink), and violet are variable according to the normal rules of agreement

2) châtain (chestnut brown) is semi-invariable - it usually agrees in number, but rarely in gender

Multiple Colors

When two or more colors describe a noun, they may or may not agree:

1) If there are items of each individual color, the adjectives agree:

2) If each item has all of the colors, the adjectives are invariable

Compound Colors

When adjectives of color are modified by another adjective or a noun, the adjectives are invariable.

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Hmmm, given the french grammar, I am afraid the girls would tell me to go away (let's remain polite...) with my french and decide to keep reading Asterix in Hebrew for ever!!!!!