1. An image color determining device comprising: a ground color presence detecting section for determining a presence of a ground color of a target image based on image data relating to the target image; a ground color region specifying section for specifying a ground color region in a color space when it is determined that the ground color is present by the ground color presence detecting section; a ground color determining section for determining whether or not the image data to be determined is a ground color region based on the ground color region specified by the ground color region specifying section; and a ground color replacing section for replacing the image data determined as the ground color region by the ground color region determining section with a predetermined color, wherein the ground color presence detecting section counts the number of constituting units of the image data for every color region on a two-dimensional color plane divided into a plurality of color regions, and determines the presence of the ground color using the counted result, and the ground color region specifying section specifies the ground color region based on three-dimensional distribution of the constituting units in the color region corresponding to the around color in the color space when the ground color presence detecting section determines that the ground color is present.