Multiple first gradation values corresponding respectively to the multiple inks are quantized to generate multiple first quantized values indicating printing or non-printing of dots with the respective inks. In this case, multiple second gradation values corresponding respectively to multiple multinary colors expressible by combinations of overlapping of the multiple inks are generated based on the first gradation values. Next, the multiple second gradation values are quantized to generate multiple second quantized values indicating printing or non-printing of dots of the respective multinary colors. Then, the first quantized values corresponding to the inks to be overlapped one on top of another to express the multinary colors are generated based on the second quantized values corresponding respectively to the multiple multinary colors.