An image forming apparatus includes a table generating unit. The table generating unit, in a Voronoi diagram, obtains empty circles as circles centered at Voronoi seeds of ends of Voronoi sides that intersect with a straight line passing through a largest saturation color and a smallest-saturation and specific-lightness color and passes through generatrices. After the table generating unit sets one of the generatrices shared by the two adjacent empty circles whose radius ratio is outside a specific range as a target point, and causes the radius ratio of the two empty circles to be within the specific range by changing at least one radius of the two empty circles, the table generating unit generates the conversion table that sets a color that corresponds to an intersection point corresponding to the target point among the intersection points of the two empty circles, as the definition colors.