1. An image-processing device comprising: a memory that stores a red range occurring within a color space; an acquisition unit that acquires image data representing a document image that includes characters; and a production unit that produces image data representing an overall image that includes: the document image represented by the image data that is acquired by the acquisition unit, a differentiation image that is positioned within the document image and that allows a user to differentiate an image of red characters residing in the range from an image formed of colors other than red, and an information image representing information for acquiring the document image in a state in which the differentiation image is not positioned.