An image forming apparatus has a sheet reading portion, an assimilation surveying portion, a color changing portion, and a printing portion. The sheet reading portion reads image data from a transparent sheet tinted with a predetermined color owned by a user. The assimilation surveying portion looks for, out of printable colors, a particular color that assimilates to the color of the read image data. The color changing portion changes, out of the data of a predetermined document accepted from the user, the color of the data of a concealable character, which is a character concealed when overlaid with the transparent sheet, to the found particular color. The printing portion prints, out of the data of the document, the data of the concealable character in the particular color.