1. An image forming apparatus comprising: a plurality of first image forming units that transfer toner images developed with colored toners to a transfer body, and form first images on the transfer body; a second image forming unit that transfers a toner image developed with a spot color toner different from colors of the colored toners to the transfer body prior to the plurality of first image forming units, and forms a second image serving as an undercoat or an overcoat of the first images on the transfer body; and a controller that controls the plurality of first image forming units in order that amounts of the colored toners used for forming the first images by the plurality of first image forming units become less than amounts of the colored toners used when the first images are formed prior to the second image.