If you want to print e.g. a photograph, you first have to separate your shot into printing and not printing dots. It goes to far to explain the details here, but it happens with a camera which has a prisma between the lens and the film that splits all the grey shades in bigger and smaller black and white dots. An illustration looks like the following (sorry for the bad scans)



Where the highlights are mostly white with some black spots and the dark shadows nearly black with a few white spots. You can see that better on the foto of Sherlock Holmes (left) and a detail of his eye (right)



For colour printing the same principle is used, but then you even need a colour filter between (or before) the lens and the prisma that "filters" the desired colour and you have to repeat this step with at least 3 different filters to be able to print the base colours yellow,magenta and cyan

Theoretically, with mixtures of these 3 colours you can print any other colour



But as no printing ink is perfect, in the most cases with 3 colours you will get only a dirty, very dark brown and that's why in modern printing also real black is used as fourth colour to give more depth. Have a look at Tom Wesselman's Bedroom Painting n° 7 with the individual colours yellow, magenta, cyan and black and the final result:



For details like in the eye of Sherlock, pay attention to the circled part of the flower in the upper left corner

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