Electric power is the product of two quantities: current and voltage.
These two quantities can vary with respect to time (AC power) or can be kept at constant levels (DC power). Most refrigerators, air conditioners, pumps and industrial machinery use AC power whereas most computers and digital equipment use DC power (the digital devices you plug into the mains typically have an internal or external power adapter to convert from AC to DC power).
Secondly, it is often more economical to install turbines that produce higher voltages than would be used by most appliances, so the ability to easily transform voltages means this mismatch between voltages can be easily managed. Solid state devices, which are products of the semiconductor revolution, make it possible to transform DC power to different voltages, build brushless DC machines and convert between AC and DC power.
To enable wires to be easily and safely identified, all common wiring safety codes mandate a colour scheme for the insulation on power conductors.
Many local rules and exceptions exist per country, state or region.1 Older installations vary in colour codes, and colours may fade with insulation exposure to heat, light and ageing. As of March 2011, the European Committee for Electrotechnical Standardization (CENELEC) requires the use of green/yellow colour cables as protective conductors, blue as neutral conductors and brown as single-phase conductors.2 The United States National Electrical Code requires a green or green/yellow protective conductor, a white or grey neutral, and a black single phase.3 The United Kingdom requires the use of wire covered with green insulation, to be marked with a prominent yellow stripe, for safe earthing (grounding) connections.4 This growing international standard was adopted for its distinctive appearance, to reduce the likelihood of dangerous confusion of safety earthing (grounding) wires with other electrical functions, especially by persons affected by red-green colour blindness. In the UK, phases could be identified as being live by using coloured indicator lights: red, yellow and blue.
The new cable colours of brown, black and grey do not lend themselves to coloured indicators.
For this reason, three-phase control panels will often use indicator lights of the old colours.Źródło: https://en.wikipedia.org/wiki/Electrical_wiring#Colour_code