Commercial aircraft are usually predominantly, if not completely white in colour. This is not because airlines are lazy to paint their fleets, but because white colour gives airplanes many advantages that make them more efficient and easy to maintain. White colour absorbs the least amount of heat and this reduces the cost of keeping airplanes cool. White colour also makes it easy for ground technicians to visually inspect the airplane and identify scratches or cracks on the body of the airplane. Painting an airplane is also an expensive project because any paint used on the outer surface of the aircraft needs to be able to withstand extreme heat and extreme cold. If an airplane is painted in different colours, the engineers will need to find colour combinations that do not fade in strong sun light or extreme cold, otherwise the airline will need to repaint the airplane frequently, increasing the time the airplane is not available for carrying passengers. One exception to painting aircraft white are military airplanes that are usually not painted in white because they need to be easily camouflaged when they are on the ground.