What is the wire color coding of ATX computer power supplies ?

Computer power supplies (PSU) convert AC to low-voltage regulated DC power. Most modern desktop computers conform to the ATX standard, which supplies three positive rails: +3.3V, +5V and +12V.

Wires coming out of an ATX PSU are color-coded as follows:

Yellow: +12V
Red: +5V
Orange: +3.3V
Black: Ground

Purple: 5V SB (standby voltage)
Green: /PS_ON (it can be shorted to ground to start PSU)
Grey: PWR_OK (status signal generated by PSU to indicate voltages are OK)
White: -5V (optional on newer ATX-2 PSUs)
Blue: -12V