1. An image forming apparatus, comprising: an image forming device to form a plurality of primary color toner images with toner of a plurality of primary colors on a latent image carrier based on input image information, and transfer the plurality of primary color toner images, directly or indirectly, from the latent image carrier to a recording sheet one above the other to form a multi-color toner image on the recording sheet; a measuring device to measure multi-colors of the multi-color toner image formed on the recording sheet to obtain multi-color information indicating the measured colors of the multi-color toner image; and a controller to: estimate primary color information of each one of the primary color toner images that constitute the multi-color toner image, based on the multi-color information obtained by the measuring device, to obtain estimated primary color information indicating estimated colors of the primary color toner images in the multi-color toner image formed on the recording sheet; and correct parameters of the image forming device that affect an image forming condition of each of the plurality of primary color toner images, based on the estimated primary color information.