1. An image processing method for generating a gradation image, the image processing method comprising: (a) acquiring a coordinate of a start point, a color value of the start point including a plurality of color component values, a coordinate of an end point, and a color value of the end point including a plurality of color component values; (b) setting either a number of pixels between the start point and the end point that is calculated on the basis of the coordinates that are acquired in the coordinate-and-color-value acquisition (a) or a color difference value of one of the plurality of color components values of the start and end points that has a largest color difference between the start point and the end point, whichever is smaller, as a number of complementary colors of a complementary color list; (c) creating the complementary color list that has the number of complementary colors that is set in the complementary-color-number setting (b); (d) generating the gradation image with the use of the complementary color list that is created in the complementary-color-list creation (c); and (e) causing the gradation image to be printed.