Disclosed is a color adjustment method for each of image forming apparatuses, including: a gradation pattern creation step of forming a gradation pattern image including patches different in gradation for each of colors in n pieces of coloring materials; a density detection step of detecting densities of the respective patches, and obtaining density information; a gamma curve creation step of associating the density information of the respective patches in a second image forming apparatus with that in the first, and creating a gamma curve correcting a gradation of received image data so as to obtain densities of the respective patches in the first image forming apparatus; and a color adjustment step of extracting respective color components, deciding output values based on the gamma curve corresponding to the second image forming apparatus, creating output image data, and forming an image on a sheet in the second image forming apparatus.