|InfoVis.net>Magazine>message nļ 184||Published 2006-10-16|
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The digital magazine of InfoVis.net
Colour plays an important, yet not indispensable, role in data visualisation as we have seen already in other articles about colour within the scope of this digital magazine.¬†
Cynthia Brewer in her page Color Use Guidelines for Mapping and Visualization proposes a set of recommendations for the use of colour in cartographic and visualisation applications that we intend to review here in summarised form¬† as a complement of the previous article.
According to C. Brewer the simplest recommendation that one usually receives about the use of colour in cartography is using hue to represent categorical differences (blue for rivers, brown for mountains green for lowlands...) and saturation to show ordered differences (lighter green, lower lands, etc.).
One of the aspects¬†that Cynthia Brewer highlights of colour is its three-dimensionality. We will not extend ourselves about the colour models that make its codification possible. On the computer world the RGB (Red, Green, Blue) model dominates. It combines the three primary elements of the cathodic ray tubes (CRTs) phosphor and later on the three filters of LCD displays. Printers use to adhere to the CMY model that refers to the primary Cyan, Magenta and Yellow inks. Many design programs use also HSB where the three elements are Hue (the hue of colour), Saturation (how pure it is, a saturated colour has no white in its composition) and Brightness (also called Value in the HSV model).¬†¬†¬†¬†¬†
The latter is closer than the others to more sophisticated perceptual models of colour and makes it simpler to specify a colour since its three dimensions refer to concepts bound to intuition, even though it is completely equivalent to RGB and both of them can be transformed one into the other very easily.¬†
In any case all of them are three-dimensional and, hence, it is of our interest to use cleverly each one of those dimensions. The users of our graphical representations don't ever think in technical but in perceptual terms regarding colour. For any of us it's simpler to describe a colur as for example "light blue" than giving the proportions of red green and blue or the corresponding CMY coordinates. Hence It's important to play appropriately with the perceptual magnitudes of colour.¬†¬†
Brewer considers 4 diferent types of colour schemes and proposes a colour system fitting each one of them:
Finally these four possibilities can be combined among them to obtain up to 10 commendable bivariate schemes. At the beginning of this article you can see an adaptation of the way Brewer presents said combinations. In this case the data has two variables that can be of any of the above mentioned types.¬†
For example we could want to represent the increase or decrease of value (diverging) in the stock market along with its volume of business (sequential). It's worth following th links of the original page of Brewer to see the multiple examples of application of these schemes. In that representation some values appear together with the colours (y, n, etc). Its only purpose is to give an example of what the values could represent. Although the matrices are 2x3 and 3x3 in dimensions it's very easy to widen the number of shades to be represented just by adding intermediate colours.
The main limitation of Brewer's colour guidelines is that it has been thought out for the representation of data variables that have a specific location in the graphic space and the colour appears precisely in that location. This covers cartographic maps, isoline maps where colour fills the space between the isolines, choropleth maps or area maps depicting categories. This limitation leaves us, nevertheless, with a wide range of applications that can benefit from the use of these guidelines.
Cynthia A. Brewer, 1994, "Color Use Guidelines for Mapping and Visualization", Chapter 7 (pp. 123-147) in Visualization in Modern Cartography, edited by A.M. MacEachren and D.R.F. Taylor, Elsevier Science, Tarrytown, NY.
See also the interesting interactive tool ColorBrewer devoted to help finding the appropriate colour scheme.
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