1. An image forming apparatus comprising: an image forming part to form an image made of a plurality of pixels by using a plurality of printing colors and a special color different from the plurality of printing colors; and a conversion part to output component data of the plurality of printing colors from input image data of the image for each of the pixels, the conversion part converting the component data of the plurality of printing colors into the component data of a multi color in which the special color is added to the plurality of printing colors to the image forming part, based on a user-specified condition for converting the component data of the plurality of printing colors into the component data of the multi color.