Before about 2001, most NM cable had a white outer jacket, but since 2001, most NM-B cable has been jacketed with a vinyl outer insulation that is colored to identify it for both consumers and inspectors. This color coding of the wire sheath is strictly voluntary, but most manufacturers adhere to the color scheme. The five basic color schemes used for the NM cable in residential construction are white, yellow, orange, black, and gray. These colors are seen in the solid vinyl outer jacket that enclose the individual conductors within the cable and are easily identified at a glance. Be aware that black is used as a color for two different wire gauges, so some care is needed when interpreting black cable. If you need black cable, make sure to read the sheathing and packaging carefully to determine if you are buying 6- or 8-gauge wire. Because the gray coding does not indicate wire size, read the packaging and jacket printing to make sure you are buying cable with the right wire gauge. The bare copper wire in the cable is the ground wire. (In rare instances, you might find cable in which the ground wire is covered with green insulation—green is the color designated for ground wires.) In a three-wire cable, there will be a red wire in addition to the white and black wires. Red is also designated as a hot color.
