Image formation adjustment method and image forming apparatus

An image formation adjustment method that adjusts an image forming apparatus, the image forming apparatus comprising:

• a developing agent supply portion configured to supply black developing agent and developing agents for a plurality of chromatic colors;
• an image forming portion configured to form an image of the black developing agent and the developing agents for the plurality of chromatic colors supplied from the developing agent supply portion on a surface of a transfer medium;
• and
• a diffuse reflection light detection portion configured to irradiate a surface of the transfer medium with light and detect an amount of diffuse reflection light reflected by the surface of the transfer medium;
• the image formation adjustment method comprising:
• a control device selecting one of the plurality of chromatic colors as a target chromatic color according to a consumption status of the developing agents of the plurality of chromatic colors;
• the control device causing the image forming portion to execute a process of forming, on the surface of the transfer medium, a plurality of chromatic color test images that are images of the plurality of chromatic colors, and a superimposed test image in which a black test image is superimposed on a background image of the target chromatic color;
• the control device acquiring from the diffuse reflection light detection portion a test detection signal that is a detection signal of the amount of diffuse reflection light in each of areas of the plurality of chromatic color test images and the superimposed test image on the surface of the transfer medium;
• and
• the control device adjusting timing of image formation of the plurality of colors by the image forming portion based on the test detection signal.