Defining characteristics of a colour
Human colour vision is essentially 3-dimensional—you need three quantities or a solid 3-D model to describe the full range of colours that most people see. You can do this in many ways. Probably the easiest system to understand intuitively is the HLS system.
In the HLS system each colour is uniquely identified by three characteristics—hue, luminance and saturation.
The RGB system is most useful in photography and computing.
The CMYK system has traditionally been used in the printing industry.
Other characteristics of a colour
Colours are described in many other ways. For example colours have traditionally been categorised as cool colours or warm colours. These characterisations are sometimes useful but they do not uniquely identify a particular colour.
Here are some examples of these descriptive characteristics: