The special curtain that covers the ark in most synagogues is called a parochet.  Often elaborately embroidered, the first reference to an ark curtain is found in Exodus 40:21, where Moses "brought the ark into the Tabarnacle and set up the curtain and screened the ark."  In the days of the First and Second Temples the parochet separated the Holy of Holies from the rest of the Temple.

It is customary to hang a white parochet on the High Holy Days, while a variety of colours is used throughout the year.