The invention provides a control method for controlling a color image forming apparatus capable of forming a color image on a transfer material (11) and provided with a first optical sensor (41) capable of detecting optical reflection characteristics of an unfixed toner image and a second optical sensor (42) capable of detecting optical reflection characteristics of a toner image after fixation, the method including a first forming step of forming a mixed-color toner image including plural toners; a calculation step of calculating, based on optical reflection characteristics of the mixed-color toner image detected by the second optical sensor, a condition that the mixed-color toner image becomes achromatic; a second forming step of forming a toner image corresponding to the calculated toner mixing ratio; and a processing step of processing an output of the first optical sensor based on the optical reflection characteristics of the monochromatic toner image detected by the first optical sensor.