1. An image processing method for converting a color signal representing a color in a first color gamut to a color signal representing a color in a color gamut of an image forming apparatus, the image processing method comprising: modifying a shape of the color gamut of the image forming apparatus according to a shape of the first color gamut so as to change the shape of the color gamut of the image forming apparatus to a shape similar to the shape of the first color gamut; and converting a color signal representing a color in the first color gamut to a color signal representing a color in the modified color gamut of the image forming apparatus based on a convergence point in the modified color gamut, wherein modifying of the shape of the color gamut of the image forming apparatus comprises deleting a part of the color gamut of the image forming apparatus which has saturation values higher than threshold values represented by a threshold line obtained by connecting maximum saturation values of the image forming apparatus' color gamut that correspond to hues of primary colors in the first color gamut, and wherein deleting the part of the color gamut of the image forming apparatus is performed for suppressing a reversal of a tonal change based on the converting of the color signal.