1. An image processing apparatus comprising: a processing circuitry that obtains a color image output from an image forming unit, the color image being formed by superimposing two or more basic colors on top of one another based on input image data, corrects gradation characteristics of the two or more basic colors to be used by the image forming unit based on a gradation correction parameter, predicts an output color of the image forming unit to generate a predicted color specification value independent from the image forming unit, extracts from the input image data a colorimetry area for color difference determination, obtains a colorimetric value of an area of the color image that corresponds to the extracted colorimetry area, using a colorimeter, and adds a colorimetric error at the colorimeter due to a color arrangement on the input image data to the predicted color specification value, to obtain a corrected colorimetric value of the area of the color image, wherein the processing circuitry corrects the gradation correction parameter to minimize, according to a prescribed function, a difference for at least a portion of the colorimetry area between the corrected colorimetric value and a color specification value obtained using the colorimeter.