Wire and cable sizes are classified by the maximum amperage that they can carry safely. A complex network of wires runs through the car. To avoid confusion, each wire is colour coded (but only within the car: there is no national or international system of colour-coding).
The side and tail lights, however, which you may need to leave on when the car is parked, are always wired independently of the ignition switch. When fitting extra accessories, such as a rear window heater which consumes a heavy current, always wire it through the ignition switch.