1. A color converting apparatus for converting a device-independent color space into a printing color space dependent on colors used in a printing machine, in color printing in which a spot color unused as a color material for the printing machine is reproduced with a plurality of color materials used in the printing machine, the apparatus comprising: a first calculating unit for calculating halftone color values of the spot color in the device-independent color space according to a certain rule represented by a line connecting, in the device-independent color space, maximum color values of a time when a dot percentage of the spot color in the device-independent color space is at a maximum, and minimum color values of a time when the dot percentage of the spot color in the device-independent color space is at a minimum; and a second calculating unit for calculating printing color values in the printing color space from the halftone color values.