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Colour and Information
by Juan C. Dürsteler [message n 95]

Colour is an integral part of most visual representations. In this issue we review the fundamental uses of colour to convey information. 

Colour is an attribute that is not strictly necessary in order to distinguish shapes or to perceive the real world in a sufficiently operative way, as anybody that has seen black and white TV or photography can state.

Nevertheless, colour is excellent to convey emotions, to label and categorise, but poor in showing shapes, details or space, where luminance (intensity) and contrast play the fundamental role. For this reason* two channels of information transmission can be considered related to colour:

  • The Luminance channel: Refers to the intensity or brightness and to the contrast. This channel dominates shape, space and movement perception. The world can be seen as just intensity variations of a single colour like B/W TV as previously said.

  • The chromatic channel. Refers to the wavelength of light, the colour itself also called the hue. Typically two main chromatic channels are considered, red-green and blue-yellow. These channels have a much lower spatial resolution but give us information about the surface of the objects. They allow us to use colour to label, convey meaning, associate to measurements, etc. It’s the symbolic component of colour, from a perceptual standpoint.
MapaEspana.gif (9645 bytes) ProgreComfort.jpg (5823 bytes)
Colour as a name. Each region has a different colour that identifies the autonomic community it belongs to. Colour as a number. Each colour corresponds t a level of astigmatism in this plot of a progressive addition lens.

If we concentrate on the chromatic channel, the most interesting for Information Visualisation, we can consider some applications of colour. Edward Tufte in his bookEnvisioning Information”   considers the four fundamental uses of colour to be:

  • Labelling. Colour as a name to code the categories of objects to be shown. On a map, for example, colours serve to label the different countries.

  • Measurement. Colour as a number. Different shades of colour can be assigned to different values of a variable like, for example, the height on a topographic chart.

  • Representation. Colour as a way to imitate reality. The blue of the rivers or the shades of the mountains confer a more realistic look to a map.

  • Decoration. Colour as beauty. An appropriate selection of colours according to their function produces an aesthetical effect.
MapaEspResalt.jpg (17564 bytes)
Colour to focus attention. The selection of an appropriate colour attracts the attention to a particular region.

But colour can be used to transmit other types of information.

  • Emotion. Colour as a way to convey mood. For not yet well understood reasons there appears to be a psychology of colour that makes, for example, red be associated with passion, life, excitation, while green is associated to nature, freshness and health.

  • Visual Attention. Colour as a way to focus attention. Among other attributes, it allow us to highlight some objects over others, while if it were only for their shape they would be much more difficult to distinguish. In this sense an adequate selection of colour makes information “pop up”.

Colour is a very broad topic that we can just touch on lightly here. The appropriate use of it is a fundamental tool for those willing to make information understandable

In the next issue we’ll get down to the detail of some of the uses of colour.

* (See the book “Information Visualization: Perception for Design” by Colin Ware, Chap. 4.)

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