1. A printing method of printing on a printing medium, comprising: providing a print head that includes a plurality of nozzle arrays for ejecting at least N colors of inks, N being an integer of at least 3; performing a halftone process with a dither matrix on image data that represents a tone value of each pixel making up an original image to generate dot data that represents a state of dot formation at each of print pixels of a print image to be formed on the printing medium; and generating the print image by combining at least N colors of dot groups formed in at least one scan of the print head in a common print area, the plurality of nozzle arrays respectively forming dot groups having different colors each other in the each scan of the print head according to the dot data, wherein the dither matrix includes a single reference matrix that is set in such a way that reduces contact among M colors (M is an integer of at least 3 and less or equal to N) of dot groups among the at least N colors of dot groups, with respect to at least some tone values among the input tone values, and the performing includes N different halftone processes, each of the N different halftone processes being performed on each of the at least N colors of inks by using the dither matrix that includes the reference matrix and (N-1) converted matrices generated by executing one of at least one type of predetermined conversion process (N-1) times on the reference matrix.