Way of printing at NPN, in which a photo or illustration is made up of black and one pms colour. A duotone effect or duplex image consists of two colours; a basic colour that lies in the background of the image and determines the characteristic colour of the image and a second colour (in all cases the darkest of the two colours) for the drawing and the actual content of the image.

The basic colour and the second colour are based on one and the same original, but must be reproduced separately from each other and in different degrees. The making of a duotone is the work of the lithographer.