An image forming system includes an image forming apparatus with attach/detach sections to/from which a developer container is attachable/detachable and an image bearing body. A driver instructs the apparatus to execute single-color or color-image formation. When a developer container is attached to each attach/detach section, the apparatus is usable as a color image forming apparatus. When a developer container is attached to only one attach/detach section, the apparatus is usable as a single-color image forming apparatus. The apparatus forms an image when the driver instructs the apparatus that is being used as the color image forming apparatus to execute single-color-image formation. The apparatus does not form an image when the driver instructs the apparatus that is being used as the single-color image forming apparatus to execute color-image formation.