1. A color name presuming apparatus for presuming a color name of a predetermined region in a color image, comprising: a unit that accepts a color name and RGB values, calculates a luminance in a YCbCr system from said RGB values, and stores said luminance into a storage device in association with said color name; a luminance calculation unit that calculates a region luminance in said YCbCr system from RGB values in said predetermined region within said color image upon detecting that said color image is a color image whose color components other than said luminance are less than a predetermined level; an extraction unit that extracts a color name corresponding to said region luminance calculated by said luminance calculation unit from said storage device; and a unit that outputs said color name extracted by said extraction unit as said color name of said predetermined region.