1. A color converting apparatus for converting a device-independent color space into a printing color space dependent on colors of color material loaded in a printing machine, comprising: an input port for receiving manuscript data for printing in the printing machine, including data calling for a spot color; one or more processors to execute color conversion for printing in which the spot color is reproduced in the printing machine not loaded with the spot color as a color material by using a plurality of other color materials loaded in the printing machine, the one or more processors configured to: calculate halftone color values of the spot color in the device-independent color space by connecting with a line, in the device-independent color space, maximum color values of a time when a dot percentage of the spot color is at a maximum, and minimum color values of a time when the dot percentage of the spot color is at a minimum; and calculate printing color values in the printing color space from the calculated halftone color values of the spot color in the device-independent color space; and an output port for transmitting the printing color values to the printing machine for printing with the color material loaded in the printing machine.