Why Printing is CMYK
The primary colours of light are known as 'additive' which just means that as you mix the colours they 'add' to one another - they become lighter.
This is quite natural if you think of a dark room, then as more lights are switched on, the brighter the room becomes.
FIGURE 1 These illustrations show the different colours produced when mixing 'additive' or 'subtractive' colours.
Where any two RGB colours mix they form the lighter secondary colours of Cyan, Magenta, Yellow (CMY). The mix of all three RGB colours creates White. So why are RGB inks not used when printing? When using ink (or paint and dyes for that matter) you are not using colours that emit light, but absorb light instead - hence they are known as subtractive colours. Red ink on paper blocks all other colours and only reflects Red light. So when Red and Blue ink are mixed the result should be Black since neither Red nor Blue survive; each ink blocks the other. In real life you do not get a true Black as inks are not pure in colour, but mixing the colours does form darker, not lighter colours.
Since the RGB colours are dark to start with, and mixing these as inks only forms yet darker colours still, then how is a lighter colour like Yellow achieved? The answer is to use light colours to begin with, and the clue as to which is in the first diagram. If Cyan (Green-Blue) is mixed with with Yellow (Green-Red) then Green is the result as it is common to both. The CMY colours (commonly mistaken for Blue, Red and Yellow) are the answer when printing as mixing any two will form one of the darker RGB colours. The mix of all three CMY colours creates Black, although in practice this is very muddy.
FIGURE 2 When an image is printed in the lighter CMY colours you would expect to get a result like the RGB image on the right. But because ink colours are not pure, the duller image on the left is the more likely result (whether you see the difference between the two images will depend on your monitor).
This muddying of the colours happens when using the primary colours of light also, but to a far lesser extent. If you wanted to get close to pure colour with light then you would probably have to use lasers as the light source. Otherwise any normal primary colour, such as Red, would still have a certain amount of Green and Blue in it as well. With inks and paint this impurity of colour becomes very noticeable. Therefore, since areas of an image that should be Black are quite obviously not so, when using CMY, then the Black has to be supplied separately as a fourth colour. This need to add Black does not reflect any condition in the nature of light; it just reflects the inability to create pure coloured inks and paints.
NOTE: The reasons for Black having the letter K are firstly that the letter B is already used for Blue and secondly that Black is often referred to as the Key colour as it is the strongest and is used to add weight and depth to an image.
FIGURE 3 Black is created artificially by removing from the CMY values those areas that the CMY mix was trying to print Black. Just how much colour value is removed is determined by the colour settings in Photoshop, or by the software that runs some higher end scanners.
There are several levels of removal that can be set which allows you quite a bit of control of Black emphasise when printing. The Black values are added back into each pixel as a fourth set of numbers; a fourth channel. It is important to understand that not one extra pixel of image detail is generated in this process; just extra numbers inserted to the already existing pixels. Converting from three to four colours makes the image one third bigger but it does not add one jot of extra image detail.
A misconception that is sometimes mentioned is that larger working file size equals extra image detail. Therefore CMYK images are, by default, more detailed than RGB images as the working file size is a third bigger. The file size part is true, but the claim that CMYK is more detailed is not. It is extra pixels (assuming they too have not been added artificially by interpolation) that make for a more detailed image, not the extra digits within the pixels.
FIGURE 4 The conversion of an image from RGB to CMYK results in a duller image with some colours affected more than others, particularly the blues. This limiting of the colour range is not inherently to do with CMY, in fact a few CMY colours are wider than in some RGB colour spaces.
The loss of colour range is to do with the technical limitation of CMYK inks not being able to be made as pure as the colour values of light. Also the contrast range in printing is limited to the black of the ink and the white of the paper which is much less than the range of an RGB computer monitor. If the printing was done using RGB inks the same tonal loss would also apply, along with all the extra problems that printing in RGB would also introduce, such as no Yellows as mentioned earlier.
It is not just printing presses that use CMYK inks, but also laser printers and inkjet printers as well. It is a common misconception that inkjets are RGB devices as they are used for printing RGB images. The inkjet print drivers will convert RGB images on the fly without you necessarily knowing it, but if you check the ink cartridges you will see that they too are CMYK. Inkjets will give better colour results than print presses because of the higher quality (and much more expensive) inks and paper being used.
Another useful link on this subject can be found here.