A color image forming apparatus and related method forms a color image on a transfer material. The apparatus is provided with a first optical sensor capable of detecting optical reflection characteristics of an unfixed toner image and a second optical sensor capable of detecting optical reflection characteristics of a toner image after fixation. A first forming step forms a mixed-color toner image including plural toners. A calculation step calculates, 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 forms a toner image corresponding to the calculated toner mixing ratio. A processing step processes an output of the first optical sensor based on the optical reflection characteristics of the monochromatic toner image detected by the first optical sensor.