1. An image forming system, comprising: an obtaining unit that obtains image information showing an image including a plurality of grey-level values of a plurality of pixels, each of the plurality of grey-level values showing a grey-level value of a pixel; a modifying unit that modifies the grey-level values of pixels included in a metallic area within the image shown by the image information obtained by the obtaining unit, the metallic area being identified as an area representing a metallic surface; a color image forming unit that forms on a recording medium a color image in accordance with information showing the color image including the plurality of grey-level values of the plurality of pixels modified by the modifying unit; and a transparent image forming unit that forms on the recording medium a transparent image overlapping the color image, wherein the modifying unit is configured to modify the plurality of grey-level values of the pixels by adding or subtracting a series of random numbers to or from the grey-level values, the random numbers being updated whenever the length of a row of pixels becomes 30-120 micrometers in a direction of the image shown by the image information.