Abstract: An opacity process for paint, including generating paint color data representing a paint color, based on the scatter and absorption data, or reflectance data, for the paint color, generating, based on the paint color data, full hiding color data representing the reflectance of the paint color at full hiding, generating, for a plurality of undercoat paints with different reflectance levels, color combination data representing the color of at least one coat of the paint color and one of the undercoat paints, respectively, and generating color difference data representing opacity, based on a difference between the full hiding color data and the color data combination.