"There are purple caps."
Translation:Il y a des casquettes violettes.
Color adjectives are regular adjectives and as such, they are placed after the noun they modify. They agree with the noun they modify.
But the agreement rule has a few exceptions:
when the color is also the name of a fruit or material: "orange", "marron" (chestnut), "argent" (silver) or "sable" (sand), etc. there is no agreement
when the color itself is modified by another adjective: bleu clair (light blue), rose pâle (pale blue), etc. there is no agreement
when the color is a compound noun with a hyphen: bleu-vert, rouge-orange, etc. there is no agreement
So now, you have to know that there are:
- exceptions of exceptions (back to the rule)
"Rose(s)" (pink + rose) and "violet(te)(s)" (purple + violet) are exceptions of exceptions! So they both agree with the noun they modify: "des chemises roses, des écharpes violettes."