1. An image processing apparatus comprising: an image generation unit that scans a document to generate image data; a hue determination unit that determines a specific hue included in the image data and a hue becoming a complementary color of the specific hue; a color reduction unit that retains the specific hue and the hue becoming the complementary color to reduce a color of the image data; and an output unit that outputs the image data whose color is reduced by the color reduction unit.