Black, white, green, red, blue, orange, brown, and gray—the colors of the insulation sheathing on electrical wires typically carry specific meanings. Therefore, when installing new light fixtures, besides turning off the circuit breaker, it's important to understand what each colored wire you encounter represents.

Residential electrical wiring in the United States initially lacked a systematic color-coding system, and there wasn’t even a standardized set of guidelines for proper usage. In 1879, shortly after Edison first introduced electric lighting, the insurance industry began issuing safety guidelines. The first formal set of guidelines emerged in 1881, covering topics such as load capacity, insulation, and installation, but did not classify wire colors.

In 1882, the National Board of Fire Underwriters (NBFU) also adopted early safety regulations. In 1893, the National Insurance Electric Association began efforts to unify the varying electrical installation rules and standards across states, proposing a national code for wiring buildings with lighting and power devices.

The first National Electrical Code (NEC) was introduced by the NBFU in 1897, which also overlooked the standardization of wire colors. Later, in 1928, the NEC was revised, and one requirement was to establish a color standard for grounding wires, which later became white or natural gray, while prohibiting these colors for live and neutral wires.

Further color-coding was introduced in the 1937 edition of the NEC, which applied color-coded wires to "multi-branch circuits" and specified that three-branch circuits should use black, red, and white wires. Additional colors, such as yellow and blue, could be used for circuits with more branches.

In 1953, the NEC changed the color for grounding wires to green or bare wire. Green was also prohibited for circuit wires (such as live and neutral wires).

The 1971 edition of the NEC streamlined the color-coding for multi-branch circuits, although white, natural gray, green, and green-yellow stripes were retained, and these colors were prohibited for grounding wires. This version abandoned rigid color-coding requirements for circuit wires due to insufficient colors to differentiate between systems, voltages, and circuits.

In the United States today, grounding wires are green, green-yellow striped, or bare; neutral wires should be white or gray; and circuit wires may be black, red, blue, yellow, orange, or brown, depending on the voltage.

These color standards are specific to the United States, and other countries follow different codes (Canada’s are very similar to the U.S.). For example, Australia and New Zealand share the same grounding wire colors as the U.S., but their neutral wires are blue or black. Additionally, live wires can be any color except those reserved for grounding or neutral wires. Red and brown are recommended for single-phase wires, while red, white, and blue are recommended for multi-phase live wires.

The United Kingdom recently (2004) updated its system to align with the International Electrotechnical Commission (IEC) standards. Their grounding wire color (green-yellow stripes) remains unchanged, while the neutral wire color changed from black to blue. Similarly, single-phase wires, previously red, are now brown. Additionally, the marking and coloring for multi-phase lines in the UK have changed: L1 changed from red to brown, L2 from yellow to black, and L3 from blue to gray.