1. A printing apparatus which performs the printing using a polish and a colorant, the printing apparatus comprising: a reduction region setting portion that sets a reduction region, which is a region where an amount of the polish is reduced, at a dark portion side of a color gamut of the printing apparatus in a device-independent color space; and a color conversion portion which converts color of an image to be input to the printing apparatus into a print color to be expressed by the polish and the colorant, wherein, among pixels constituting the image, in a pixel included in the reduction region, the color conversion portion reduces an amount of the polish compared to a pixel not included in the reduction region, and converts color of the image to the print color, wherein the reduction region setting portion has: an acquisition portion which acquires a plurality of intersection points obtained for each of a plurality of hues in which a relation line indicating a relationship between a color gradation of a printing region of the case of performing the printing only using the colorant and an index value relating to a color brightness intersects with a relation line indicating a relationship between a color gradation of a printing region of the case of performing the printing using the colorant and the polish and the index value; a three-dimensional shape determination portion which determines a predetermined three-dimensional shape in the color space, based on a plurality of reduction region reference points which are points corresponding to the plurality of intersection points in the color space; and a region determination portion that obtains an overlap region between the color gamut and an inner portion of the three-dimensional shape and determines the overlap region as the reduction region.