None. The W3C/CSS named-color set has no entries beginning with Z — making it one of several letters (with E, J, Q, U and X) that contribute no web-standard color names.

Zaffre (#0014A8) is a deep cobalt blue made by roasting cobalt ore. First recorded as a color name in the 1550s, it was used to prepare smalt and to stain glass blue.

Zinzolin (#6C0277) is a deep reddish-violet, a historical dye color whose name dates to 17th-century France.

Zinnwaldite (#EBC2AF) takes its name from a pale mica mineral, while Zomp (#39A78E) is simply a coined name for a muted cyan-green on the color lists.