An image forming apparatus (typically a color printer) and control method use a detachable yellow toner process cartridge Y for forming a yellow toner image on a recording medium and a detachable magenta process cartridge M for forming a magenta toner image on the recording medium after the yellow image has been formed. By detecting the amount of toner user by the yellow process cartridge Y and estimating the amount of yellow toner that has gotten into the magenta process cartridge M, the apparatus and method can determine when the useful life span of the magenta process cartridge M is at an end. By displaying a message to a user that it is time to replace the magenta process cartridge M before print picture quality deteriorates, the apparatus and method make it possible to avoid outputting poor quality printed images.