Input color values corresponding to an input image are received; a color space of the input color values is converted to an LMS color space with three elements, the three elements including a long wavelength (L) element, a medium wavelength (M) element, and a short wavelength (S) element; color values in the LMS color space are grouped into a plurality of groups based on color vision information, each of the groups including color values determined as approximate based on the color vision information; and lightness of color values included in each of the groups is adjusted and an image output device is instructed to apply said adjusted lightness of color values to said input image.