1. An image forming apparatus comprising: a sheet-type determining unit configured to determine a sheet type of a recording sheet; a sheet-characteristic measuring unit configured to measure a sheet characteristic of the recording sheet; a storing unit configured to store therein setting information including a save rate which is associated with one or more sheet characteristics for each sheet type, the save rate indicating a ratio of a reduction amount of color material for an image on the recording sheet to a consumption amount of the color material for an image on a reference sheet and the save rate being a value determined based on an amount of information relating to an appearance obtained from an output result on the recording sheet and the reference sheet; an image processing unit configured to determine the save rate corresponding to the sheet type and the sheet characteristic of the recording sheet from the setting information, and perform the image processing on image data of an image to be formed on the recording sheet on the basis of the determined save rate; an adjusting unit configured to adjust the save rate based on a difference between the measured sheet characteristic of the recording sheet and the sheet characteristic of the reference sheet; and an image forming unit configured to form the image of the image data processed by the image processing unit on the recording sheet.