Learn French: Ces robes violettes sont chères. - These purple dresses are expensive.

The masculine form is violet. Normally, we just add an -e to make an adjective feminine. But adjectives ending in -et are irregular: you have to double the final consonant to make -ette (violette). Then we add an -s because robes (dresses) is plural.

Why doesn't chères double its final consonant too?

Adjectives ending in -er, like the masculine cher (expensive), have their own special rule. Instead of doubling the consonant, you change the ending to -ère to make it feminine (chère). Since robes is plural, it becomes chères.

Does the double T change how we pronounce the color?

Yes! In the masculine word violet, the final t is silent. By doubling the consonant and adding an e to make violettes, you force that t to be pronounced. The plural -s at the very end stays silent, though!

Why does ces go before the noun, but violettes goes after?

Ces (these) is a demonstrative word used to point things out, and those always go before the noun. Violettes describes a color, and in French, color adjectives almost always go after the noun.