An image forming system includes an electrophotographic image forming apparatus which forms an image on a sheet of paper, and a paper coloring apparatus which is connected to the electro-photographic image forming apparatus. The paper coloring apparatus includes a coating apparatus having two coating rollers which coat colored water on both faces of the paper sheet on which the image has been formed by the electro-photographic image forming apparatus. A supply section selectively supplies one of the colored water and a colorless water onto the two coating rollers of the coating apparatus. And a controller is provided for switching between a coloring process to color the paper sheet by supplying the colored water onto the two coating rollers, and a humidifying process to humidify the paper sheet by supplying the colorless water onto the two coating rollers.