Image forming system and method for forming a color image on recording medium and for forming a transparent image overlapping the color image on a recording medium

An image forming system includes: an obtaining unit that obtains image information including a grey-level value of a pixel; a modifying unit that modifies the grey-level value of the pixel included in the image information obtained by the obtaining unit; a color image forming unit that forms on a recording medium a color image in accordance with information showing the color image including the grey-level value of the pixel modified by the modifying unit; and a transparent image forming unit that forms on the recording medium a transparent image overlapping the color image.