1. An image forming apparatus comprising: a plurality of detachable image forming devices each configured to form a color image; an image carrying device configured to carry the color images sequentially overlaid into a single color image; an exchange detecting device configured to detect an exchange operation of the plurality of detachable image forming devices; a test pattern reading device configured to read a predetermined test pattern formed by the plurality of detachable image forming devices on the image carrying device; and a controller configured to instruct the plurality of detachable image forming devices to form the predetermined test pattern on the image carrying device when the exchange detecting device detects the exchange operation, and to perform a color image position adjustment based on readings of the predetermined test pattern by the test pattern reading device.