1. An image forming apparatus including a fixing unit configured to heat and fix a toner image transferred onto a printing material, comprising: a switchback mechanism configured to reverse a conveyance direction of a printing material on which the toner image is fixed; and a colorimetry unit which is located in a vicinity of a conveyance path conveying the printing material, the conveyance direction of which is reversed by the switchback mechanism, and is configured to obtain pieces of color information of patches of a plurality of colors, formed on the printing material, from light reflected by the patches of the plurality of colors upon irradiating the patches of the plurality of colors with light, wherein an average of a variation in the color information of a patch, formed in a first region on the printing material, in response to a predetermined change in temperature is larger than an average of a variation in the color information of a patch, formed in a second region on the printing material, in response to the predetermined change in temperature, and the first region and the second region are on a leading edge side and a rear edge side, respectively, at a time of passage through the fixing unit when the printing material is divided into two regions in a direction perpendicular to the conveyance direction.