A colour image or video can be represented by the red, greeen and blue (RGB) primary colours added together in appropriate proportions, but this doesn't work for printing, since each colour ink subtracts a range of colours from the incident white light rather than adding anything. So printers use the complementary colours Cyan (green + blue, absorbing red light), Magenta (blue + red, absorbing green light) and Yellow (red + green, absorbing blue light). In practice, printers tend to use black ink as well, and this is referred to as CMYK.