1. A color conversion method for a color image device, characterized by comprising: a color data input step of reading therein color device values and measured color data thereof in the color image device; a Delaunay diagram constructing step of constructing a Delaunay diagram representing a color gamut of the color image device by subdividing the color gamut of the color image device including the measured color data distributed in a three-dimensional color space using Delaunay tetrahedrons and deleting Delaunay tetrahedrons formed on a concave surface of the color gamut of the color image device; a Delaunay tetrahedron searching step of searching for a Delaunay tetrahedron including given input colors through the Delaunay diagram; and a color device calculating step of calculating color device values corresponding to the input colors using relative positions of the input colors in the Delaunay tetrahedron and color device values corresponding to four vertices of the Delaunay tetrahedron.