To provide an image processing apparatus and its program by which a limited colorization process can be performed where a lack of information and allochroism are suppressed through the replacement to representative colors.

A representative color obtaining unit 11 obtains a plurality of the representative colors. A difference vector calculation unit 21 in a pixel value replacement unit 12 determines an average color in a peripheral region of a processing target pixel in an input image to calculate the difference vector which is directed from the average color to a color of the processing target pixel and is on a color space. A representative color selector 22 determines the length of the difference vector calculated in a difference vector calculation unit 21. Then, when the length of the difference vector is longer than a given threshold, in the difference vector direction a representative color nearest to a straight line which passes the difference vector is selected. On the contrary, when being shorter, a representative color nearest to the color of the processing target pixel on the color space is selected. A replacement unit 23 replaces the color of the processing target pixel by the representative color selected by the representative color selection unit 22.

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