1. An image forming apparatus comprising: an acquisition unit configured to scan a sheet that is printed with decolorable color material to acquire a color image of the sheet; and a controller configured to convert, using parameters stored in a storage unit, the color image acquired by the acquisition unit to a monochrome image with a specific color that is a color of the decolorable color material being darker than other colors, wherein the controller computes a pixel value of the converted monochrome image using R i ×D 00 +G i ×D 01 +B i ×D 02 +R i 2 ×D 03 +G i 2 ×D 04 +B i 2 ×D 05 +R i ×G i ×D 06 +G i ×B i ×D 07 +B i ×R i ×D 08 +D 09 where R i , G i , and B i are a red value, a green value, and a blue value, respectively, of a pixel of the color image acquired by the acquisition unit, and D 00 to D 09 are the parameters.