In printing technology, a lithographic printing method uses three primary colors (red, yellow and blue) and black pigment (ink or dye) to realize full-color reproduction according to the principle of subtractive mixing.

So many colors can't be printed, and there are many limitations. However, sometimes printing needs to achieve certain results. When printing PS plates, add several spot color plates, adjust the colors to be printed in advance, and use the printing machine several times to complete it. So five-color printing and six-color printing appeared.

If the original color is reproduced with ink other than yellow, magenta, cyan and black ink, it should be called spot color printing or spot color printing, not four-color printing.