In the RGB system, colour is defined in terms of chromatic light: that is, a mixture of red, green and blue light which, when combined, produces all the colour-light like those on the screen or the lights of a theatre scenario. Combining those three values we obtain all the possible colours. A value of (0,0,0) is black, the maximum value in each one yelds white light.
Another system with three parameters to define the colour is the HSV system (of hue, saturation, value ). The three parameters are related to those of system RGB. The dye (hue) is the colour to begin with; saturation, intensity or chroma means what concentration the pigment has, and the value supposes a more or less dark tonality.
The CMYK System, of Cyan, Magenta, Yellow, BlacK, defines the colours in additive mix, as an inkjet printer or the commercial press does. The colour is the result from the superposition of semitransparent dots of the four colours cyan (a kind of blue), magenta (intense pink colour), yellow and black. The system is additive, unlike the previous ones, and here (0,0,0,0) means pure white (usually the blank paper.)
Colour in screen and printed is very different, and the range of colours that can be represented doesn’t match exactly. The colour management systems allow at least that the colours simulated in screen are as approximate as possible to those which will be obtained at the time of printing.