Colors
Colour matching is far from simple in French. However, the following cases can be distinguished.
As a general rule, when colour is an adjective, it agrees in gender and number with the noun.
des étoffes
des souris vertes
Where it gets complicated is when the adjective in question is a common noun that is taken adjectively. Basically, the adjective is also a noun. At that point, the colour is invariable.
des yeux marron (de la couleur du marron)
des étoffes orange (de la couleur de l'orange)
Other noun that can be used to express a colour and that are invariable include: abricot, ardoise, argent, azur, brique, bronze, café, caramel, champagne, chocolat etc.
However, to put it simply, some nouns are assimilated to adjectives and therefore fit the qualified noun. These are : mauve, pourpre, rose, écarlate, fauve, vermeil et incarnat.
des étoffes mauves
Two adjectives used to designate a single colour are invariable.
des yeux bleu clair (without dash)
If they are two colored adjectives, we hyphenate.
des yeux bleu-vert
We write "des balles rouge et bleu" if each ball has red and blue. But you write "des balles rouges et bleues" if there are only red balls and only blue balls.