1. A gradation image forming apparatus comprising: a gradation data generation section which generates gradation data in each pixel of a gradation image based on a reference color table including at least one reference color number corresponding to a pixel value of a reference color which is a start color of the gradation image, a difference table including at least two difference numbers corresponding to differences in pixel values between adjacent pixels in the gradation image, and a pixel value of each pixel of the gradation image.