1. An image processing apparatus comprising: a colorimeter configured to obtain a colorimetric value of an area of a color image; a memory having computer readable instructions stored thereon; at least one processor configured to execute the computer readable instructions to, obtain the 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, correct gradation characteristics of the two or more basic colors to be used by the image forming unit based on a gradation correction parameter, predict an output color of the image forming unit to generate a predicted color specification value, extract from the input image data a colorimetry area for color difference determination, obtain a colorimetric value from the colorimeter of an area of the color image that corresponds to the extracted colorimetry area, add a reading error that is introduced by the colorimeter to the input image data during the obtaining of the colorimetric value by the colorimeter to the predicted color specification value, to obtain a corrected obtained colorimetric value of the area of the color image, correct the gradation correction parameter to minimize, according to a desired correction function, a difference for at least a portion of the colorimetry area between the corrected obtained colorimetric value and a detected color specification value obtained using the colorimeter, and derive a second reading error based on the desired correction function, the desired correction function corresponding to a flare correction function, and includes the derived second reading error in a color specification value for the colorimeter, to correct the predicted color specification value.