Custom Painted Passenger Airplanes

These planes are the exception but have you ever thought Why are planes nearly always painted white? There are exceptions. Air New Zealand once turned a Boeing 777 into a giant advert for The Lord of the Rings. Mango, based in Johannesburg, utilises a bright orange hue, while Siberian carrier S7 tends to colour its planes lime green. But the vast majority of passenger aircraft are painted white. Why?