Definition: Colour Depth

Definition: Colour Depth | Glossary entry

Colour Depth

This is about the levels of colour that are found in a graphics display. There are many possible variations of Red, Green, Blue (RGB). There are also the full range of colours derived from these three and the variations and brightnesses available. Colour depth involves huge numbers of colours.

Colour depth is expressed in ‘bits’. This is a computing term that describes the number of possibilities that are available for storage (normally as a binary number). We talk about colour depth as having a bit-colour depth. The following table shows how the different colour depths match to the different colour graphics systems…

  • 1-bit colour (21 = 2 colours): monochrome, few tones, often black and white.
  • 2-bit colour (22 = 4 colours): CGA, gray-scale – early computers.
  • 2 bits (4 colours) Some colour displays for machine readouts.
  • 3-bit colour (23 = 8 colours): many early home computers with TV displays (ZX Spectrum and BBC Micro)
  • 4 bits (16 colours) Early colour displays.
  • 4-bit colour (24 = 16 colours): as used by EGA and by the VGA standard graphics systems.
  • 7-bit colour (27 = 128 colours): Pre-EGA systems.
  • 8-bit colour (28 = 256 colours): early colour Unix workstations, VGA at low resolution, Super VGA, colour displays. 3 bits (8 possible levels) for each of the R and G components, and the two remaining bits in the byte pixel to the B component (four levels), enabling 256 (8 × 8 × 4) different colours.
  • 16-bit colour (216 = 65536 colours): XGA, (during the 1990’s)
  • 24-bit colour “True Colour” (512 = red/green/blue colour scheme) modern LCD screens, camera screens and plasma screens.
  • 48-bit colour or “Deep Colour”. Deep colour supports 30/36/48/64-bit for three RGB colours. This system supports a total colour variation measured in billions of colours. Video cards with 10 bits per one colour (30-bit colour RGB), started coming into the market in the late 1990s. It is now an established standard.

Colour depth is related to the total number of storage positions in a memory chip. The storage does not just hold a total number of possible colours, it holds variations of shade and hue for each of the colour components. So in a 24 bit system there are 256 colour variations for each of the three colours. Thus, in such a system there is 256 x 256 x 256 colours/variations of each colour… a total of 16,777,216 colours. This should be contrasted against the fact that the human eye can see about 10 million colours. As the the number of colours possible exceeds the number we can see the difference is made up with brightness variations of the colours.

The operational colours (RGB in this case) and the brightness variations that can be seen are not the only way the format is used to describe colour depth. The RGB (red/green/blue) colour format is used for cameras and a number of other media technologies. However, the colour mixtures of the RGB format create a broad range of colours. In terms of colour depth the broad range of colours produced by the mix is called the Gamut. The full description of the colour depth includes the not only the hues range, RGB itself, but also the other derivative colours.