1. An image forming apparatus, comprising: an image forming device configured 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 configured 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 configured 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, the estimated primary color information indicating an estimated output halftone ratio of each one of the primary color toner images that constitute 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; a storage device configured to store, for each of the plurality of primary colors, association information indicating association between a plurality of halftone ratios and a plurality of spectral reflectance distributions each obtained from a primary color toner image formed with the corresponding halftone ratio, wherein the controller is further configured to: generate, for each one of the plurality of primary color toner images, a plurality of candidate values of the estimated output halftone ratio of the primary color toner image based on the input image information, generate, for each one of the plurality of primary color toner images, an estimated spectral reflectance distribution that corresponds to each one of the plurality of candidate values of estimated output halftone ratio, using the association information stored in the storage device, select, for each one of the plurality of primary color toner images, one of the plurality of candidate values of estimated output halftone ratio that can minimize a difference between the estimated primary color information obtained based on the estimated spectral reflectance distributions of the primary color toner images that constitute the multi-color toner image, and the multi-color information of the multi-color toner image obtained by the measuring device, and define, for each one of the plurality of primary color toner images, the selected one of the plurality of candidate values of estimated output halftone ratio that can minimize the difference, to be the estimated output halftone ratio.