1. An image forming device comprising: an image forming unit that forms an image onto a recording medium, the image comprising at least one of a test image and a user-requested image; a controller that controls the image forming unit to form a test image including a mixed color on the recording medium; a transporting path that transports the recording medium on which the test image is formed by the image forming unit; an image detecting unit disposed on the transporting path at a position downstream of a fixing unit, the transporting path being a discharging path between the fixing unit and a discharge opening for discharging recording mediums to outside of the image forming device, the image detecting unit detecting the test image formed on the recording medium; and a calibration unit that performs a color calibration process based upon the test image detected by the image detecting unit, wherein the image forming unit includes an optical scanning device that applies light to a photoreceptor to write an electrostatic latent image and the calibration unit changes a quantity of light applied by the optical scanning device, and a substantially achromatic color is selected as a color included in the test image, from mixed colors set to be used for printing processes following the forming of the test image, based on the substantially achromatic color having a high frequency of occurrence.