Artists refer to colour using three attributes, hue, saturation (or intensity) and luminosity (or value). Hue answers questions such as "Is it red?". Saturation describes a colour as bright or dull, while luminosity points to whether it is light or dark.
Colour is a very subjective perception. What is green or yellow or blue or purple very much depends on the observer. Even if two people looking at a wall agree that it is white, still neither one of them will ever know whether both see the same thing. And the red of Georgia O'Keffee's poppies or the blue of Van Gogh's Starry night speak to us also of the artist's emotional meaning. However, if colours are to be displayed on a computer monitor for instance, they have to be understood by a device far removed from any subjective sense of emotion, a device that is able to process data only by means of very specific commands. So, how does a computer deal with the colours we see on the monitor's screen?