An image forming apparatus is a color converting device for converting a first color value acquired from printing data to a second color value. A color position calculating section calculates a hue plane and a chroma of the first color value. A color value converting section converts the first color value to the second color value using a K-monochrome color conversion table in which achromatic colors are set as monochromatic colors when the chroma of the first color value calculated by the color position calculating section is greater than a chroma threshold value γ, and using a hue plane color conversion table in which the achromatic colors are set as mixed colors of a plurality of colors when the chroma is not greater than the chroma threshold value γ.