An image data processing apparatus includes a white-colorant-amount specifying unit that specifies an amount of a white colorant used by an image forming apparatus to form an image having a unit area, the image forming apparatus forming an image on a recording medium by using the white colorant and at least one process-color colorant; and a process-color-colorant-total-amount adjusting unit that adjusts a total amount of the at least one process-color colorant used by the image forming apparatus to form the image having a unit area. The process-color-colorant-total-amount-adjusting unit adjusts the total amount of the at least one process-color colorant so that a sum of the amount of the white colorant and the total amount of the at least one process-color colorant used to form the image having a unit area is constant.