An assembly of prefabricated panels is provided including a plurality of prefabricated panels and mounting brackets, each of the mounting brackets having a strip structure and a plurality of hooks arranged such that the hooks are uniformly spaced apart. A predetermined range having the obtained hue at a center thereof is set as a conversion target range, and at least one of saturation, brightness, and hue is changed with respect to a pixel of the display image which has a hue within the conversion target range to display a resultant image on the display device. A fly height calibration track is then read to set a fly height reference.