It has been emphasized above that the RGB model is additive. It is well adapted to situations like computer screens because the screen emits its own light. But can’t be applied to printed images. This can be explained easily.

When an image is printed we start with a white sheet of paper and we print a colored dot: it absorbs the light in a given range of wavelengths; now we print another dot of another color: it absorbs light in another range of wavelengths. You see clearly that the printing of an image is a subtractive process. Another color model is needed.

It is the case of the CMY (cyan, magenta, yellow) colorspace. Basically printers use 3 inks of the colors listed above. Cyan absorbs red wavelengths, magenta absorbs green and yellow absorbs blue. If an area is totally covered by an equal deposit of the three inks it must look black (the subtractive process) and by controlling the amount of each ink (printed as dots on the paper) it is possible to reproduce all the colors.

Well, that is the theory but in the real world there are some drawbacks. First of all the CMY colorspace can’t cover the whole range or colors and brightness which can be obtained by the RGB technique. This is why images with very saturated colors and high contrasts look often somehow disappointing when printed on paper.

Secondly real pigments are not perfect: for example cyan mainly absorbs red, but may also absorb a small amount of other wavelengths. This explain why some shifts in colors are observed. Indeed if you try to obtain black by mixing the three CMY inks, the result is a dark brown.

This problem has been known for a long time and is corrected by the addition of a fourth ink, black, which can be mixed with the three others to obtain darker colors. This technique is known under the name of quadrichromy, or CMYK model and it gives very good results in classical printing.

These shifts of colors are much more difficult to correct with inkjet printers because inks are very different from those used by classical printers. Most of the inkjet printers use inks which are colored solutions. Some expensive inkjet printers can use two more cartridges of special inks for more photo realistic results but this may not be enough to render some fractal images. A few printers use inks which are suspensions of pigments. These inks are expected to be more stable and can give more saturated  colors.