1. An image forming apparatus which performs image formation of a target image with the use of a first color gamut as a standard color gamut for image formation, comprising: a storage section configured to store a color expansion table indicating a second color gamut wider than the first color gamut; a color conversion section configured to convert, when a color in the target image is present outside the first color gamut, a color value of the color in the target image to a color value of a coloring material used for image formation on the basis of the color expansion table; and an image forming section configured to perform image formation of the target image on the basis of the color value of the coloring material, wherein the storage section stores a plurality of the color expansion tables respectively indicating one of the plurality of the second color gamuts, each of the second color gamuts is made correspondence with a second coloring material amount as an amount of coloring material necessary for expressing the corresponding second gamut, the image forming apparatus further comprising: a color table selection section configured to select, when the color in the target image is present outside the first color gamut, one color expansion table from one or more color expansion tables each of which indicates the second color gamut, which includes inside thereof the color outside the first color gamut, out of the plurality of the second color gamuts on the basis of the second coloring material amount, wherein the color conversion section converts the color value of the target image to the color value of the coloring material on the basis of the color expansion table selected by the color table selection section.