1. An image processing apparatus comprising: a reference pixel determination unit configured to determine a reference pixel by scanning first image data, wherein saturation of the reference pixel is larger than a predetermined threshold; a color space division unit configured to set a plurality of regions in a hue circle by determining a region of the hue circle that a saturation is smaller than the predetermined threshold and by dividing, in a hue direction, the region of the hue circle that a saturation is larger than the predetermined threshold based on the hue value of the reference pixel determined by the reference pixel determination unit, and configured to set a plurality of luminance levels by dividing a luminance direction, and configured to determine a plurality of bins based on the set regions of the hue circle and the set luminance levels; a frequency distribution generation unit configured to generate a frequency distribution by counting a number of pixels that belong to each of the bins determined by the color space division unit; a determination unit configured to select N bins based on the generated frequency distribution and configured to determine N types of pixel values respectively corresponding to the selected N bins; and a substitution unit configured to substitute each of pixels included in the first image data using the determined N types of pixel values.