1. An image forming apparatus, comprising: a processor; and a memory storing a computer-executable program, wherein, when the computer-executable program is executed, the processor is configured to function as; an image process part that creates image data including pixel values of a process color image and pixel values of white which becomes a base of the process color image based on image forming data for forming an image with predetermined colors; and an image forming process part that forms the image on a medium using a process color developer and a white developer based on the image data created in the image processing part, and the image process part; determines a black portion in which a combination of the pixel values of the process color image is regarded as black and a non-black portion in which the combination of the pixel values of the process color image is regarded as a color other than black, and identifies the pixel values of white for the base of the process color image in the image data such that an usage amount of the white developer for the black portion is less than that for the non-black portion.