This article describes what a colourspace is, and the components which make up a colourspace.
What is a colourspace?
A colourspace is a set of parameters which define the colour capabilities of a particular device or stored in a digital file, generally outlined by a set of three colour primaries and a white point.
Each monitor and file can display / store a certain range of colours, which is known as its colour gamut.
For example if you have two displays with different colourspaces, and you look at the same file on different devices, the output from the displays will be different as one device may not be physically capable to displaying the same colours as another display.
Using specialized hardware, you measure the various aspects which make up a displays output that can be simplified to its red output, its green output, its blue output, known as its colour primaries, and its white point which is the point of the purest and lightest white in a colourspace.
However, just having these values doesn't help much as they can all be arbitrary depending on the hardware used to measure them, so standards are used to make all the values comparable.
The most common standard is the CIE 1931 Chromaticity Diagrams and more information about this can be found here: Q100330: Generating chromaticity diagrams
This CIE 1931 Chromaticity standard diagram appears to be the most commonly used standard to compare colourspaces of displays and files for the human eye. Each colourspace will cover a range from this diagram, with some being bigger that the CIE range. However, using colours outside of this range has very little value as the average human eye cannot see those colours.
The image below shows some examples of different colourspaces when compared to the CIE 1931 Chromaticity standard, such as ACES2065-1, ACEScg, DCI-P3Rec, Rec 709 and Rec 2020:
The points of the triangles for these colourspaces are defined by the colour primaries and the dot in the middle of the triangle is the white point.
For example, the Rec 709 primaries and white point as defined by the ITU-R Bt.709-6 are:
The source of the table above can be found in the ITU-R documentation.
These colourspaces are then used to calculate how every value of RGB should look like to the average human eye.
Why are there so many colourspaces?
Colourspaces are standards which when they have been defined, cannot be changed. So sRGB, for example, was developed by HP and Microsoft for use on CRT monitors, printing and on the internet, and as such has a range of values that meet what a CRT monitor can output.
However as technologies develop, users need access to a wider range of colour values so newer standards are developed to encompass those requirements, such as rec2020 for HDR content.
Other colourspaces are defined by the physical attributes of the recording devices, so RED for example have a range of colourspaces for their various camera models depending on the capabilities of the cameras hardware.