The diagram below, known as CIE 1931 chromaticity diagram, represents all the colors visible to the human eye.

The peripheral devices do not have the same ability to "see" colors as the human eye. Input devices (scanner, digital camera) cannot read all the colors that the eye can see, and the output devices (monitor, printer, video recorder) cannot reproduce them all. Each device can only read or reproduce a part, or subset, of the range of colors visible to the human eye.This is that particular device's so-called gamut of colors.

Different types of monitor use different types of phosphors (even if made by the same vendor), and thus have different gamuts. Even two monitors of the same model, made one immediately after the other, have different gamuts. Furthermore, the same monitor's gamut will gradually change as it gets older, as well as as a result of adjustments to contrast and brightness.

The RGB colors on the monitor are expressed by means of three numbers with values ranging from 0 to 255. For example, the red that you can see here to the left has the coordinates R = 255, G = 0, B = 51. Bear in mind, however, that these same numbers will produce (slightly) different colors on different monitors.

Thus, every single monitor has its own particular gamut of colors, its own set of producible colors, its so-called color space, which is, as we have seen, of RGB type (i.e. produced by additive mixture of light from red, green and blue phosphors).

Rather than a single RGB space for all monitors, there is a whole family of RGB spaces for monitors, each a little different from the next. In other words, the RGB space depends on the monitor in use (or more generally on the device: it is device-dependent). It is thus not a single space, but one for every device.

In the illustration below, one of the triangles inside the chromaticity diagram represents the colors that a particular monitor can produce. The colors producible on another monitor is shown with a differently positioned triangle.

Note that a RGB gamut in the CIE 1931 chromaticity diagram is represented with a triangle whose vertices are R, G and B.

An offset printer produces colors by layering semi-transparent inks on top of one another. Were such inks perfect, three would be sufficient: cyan blue (C), magenta (M) and yellow (Y). In practice, a fourth is required: black (K), The layering of these inks creates the colors by subtractive synthesis (every ink subtracts something from the white of the paper). In addition, the images are printed as dots of ink and the proximity of the dots also produces colors by additive blending.

The printer’s colors are expressed by four numbers, with values ranging from 0 to 100, which indicate the CMYK ratio of ink in each pixel. However, a specific CMYK percentage will give different colors on different printers.

Each printer has its own gamut; that is, its own color space, which in this case is a CMYK space (i.e. produced in subtractive mixture by cyan, magenta, yellow and black inks).

Different types of printer use different types of ink (and different paper, different ways of adding black and different grades of screen) and thus have different gamuts. One particular printer's gamut will also be altered by changes to the inks, the paper, or to other characteristics.

Therefore, there is no single CMYK space for printers; there are many spaces, one for each print combination (printer, inks, paper). As with RGB, the CMYK space also depends on the device (i.e. the printer) in use.

Note that a CMYK gamut is represented, in the CIE 1931 chromaticity diagram, with a shape which is not a triangle (roughly spoken, it is a triangle with vertices in C, M, and Y and with rounded sides).

The color gamut of a printer is different from that of a monitor, although both are subsets of the chromaticity diagram (i.e of the set of colors visible to the human eye). The gamut of a printer is normally narrower than that of a monitor. In other words, the printer can only reproduce a part of the gamut of a monitor. Depending of the printer/monitor combinations, there are also colors which can be printed but cannot be produced by the monitor.

Generally speaking, each device has its own gamut, and the various gamuts of the various devices, as depicted in the CIE diagram, overlap and intersect with one another. This means, for example, that there are some colors which can be seen on a particular monitor but which cannot be printed, and others which can be printed but cannot be viewed on the monitor; there are colors which can be seen on one monitor but not on another, or which can be picked up by a particular scanner, but cannot be printed on a particular printer... and so on.