What colour or metal is the heraldic tincture Gules?

The heraldic name Gules, on a coat-of-arms or flag, is red. The name comes from Persian "gul", rose, and is one of the oldest terms in heraldry. "Gules plain" is the arms of French family d'Albret, and gules is one of the most popular tinctures for coats-of-arms and flag design, both as the field colour and that of the devices.

When displaying the coat-of-arms as a black-and-white engraving, Gules is represented as vertical lines.