An image forming apparatus includes a luminance information acquiring unit, a hue information substituting unit, and a color conversion unit. The luminance information acquiring unit is configured to acquire input luminance information of inputted multi-gradation monochromatic image data. The hue information substituting unit is configured to substitute the acquired input luminance information with hue information to generate hue data. The color conversion unit is configured to convert the hue data into a color space for printing to generate multi-gradation color image data.