1. A color adjustment method for performing color adjustment for each of plural image forming apparatuses, the color adjustment method comprising: a gradation pattern creation step of, in each of the plural image forming apparatuses, forming, on a sheet, a gradation pattern image including patches different in gradation for each of colors in n (n is a positive integer) pieces of coloring materials, the colors ranging from a primary color composed of one color to an n-ary color composed of n colors in the n pieces of coloring materials; a density detection step of detecting densities of the respective patches included in the gradation pattern image formed on the sheet, and obtaining density information for each of the image forming apparatuses; a gamma curve creation step of, among the density information of the respective patches for each of the image forming apparatuses, the density information being obtained in the density detection step, associating the density information of the respective patches in a second image forming apparatus other than a first image forming apparatus serving as a reference with the density information of the respective patches in the first image forming apparatus, and then creating, for each of the image forming apparatuses, a gamma curve corresponding to each of the respective colors ranging from the primary color to the n-ary color, the gamma curve serving for correcting a gradation of received image data, so as to obtain densities indicated by the density information of the respective patches in the first image forming apparatus; and a color adjustment step of extracting respective color components ranging from the primary color to the n-ary color from the image data inputted to the second image forming apparatus, deciding output values in each of the respective color components based on the gamma curve corresponding to the second image forming apparatus that has received the image data, creating output image data by synthesizing the decided output values of the primary color to the n-ary color with one another, and forming an image on the sheet in the second image forming apparatus based on the created output image data.