1. A device for converting a color image including multiple character images into a monochrome image, comprising: a color conversion unit configured to reduce the image density of black character images included in the color image to a level lower than the image density of specific color character images included in the color image and having a specific color other than black during the conversion of the color image into the monochrome image, and a character color mix determination unit configured to determine a combination of the black character images and the specific color character images in the color image which have the same character properties excluding color, wherein the color conversion unit is configured to perform a process of reducing the image density of the black character images to a level lower than the image density of the specific color character images only for the determined combination.