There are many different types of enamel including cloisonne, champleve and painted enamel. In the Art Deco era, vitreous enamel was popular. This technique involved fusing glass over metal which was usually engine turned. Clock and watch faces were usually enameled and items such as vanity sets and cases were also popular.
The color of enameled objects is obtained by the addition of various minerals, often metal oxides cobalt, praseodymium, iron, or neodymium. The latter creates delicate shades ranging from pure violet through wine-red and warm grey. Enamel can be transparent, opaque or opalescent (translucent). Different colors can be mixed to make a new color, in the manner of paint. Reference: Wikipedia
Below are some examples and price guides of Art Deco enamel items including an Art Deco card case and a “Dog of Fo” rose quartz table clock.