Conceiving and Reasoning Visualization Design Options - Data Visualization: A Successful Design Process - page 94

The next image is an example of where preprogrammed understanding of color
representation can be utilized. In this case, we see the respective strength of party
political support across the U.S., with the Republicans represented by their established
red and Democrats in blue. This is a topological map that displays calculated contours
to show the general spread of support for each party. An added dimension to this
particular piece is the use of an extra representation—color transparency—to
represent population density, thus adjusting the display to accommodate the lack
of population uniformity.
Image from "Isarithmic Maps of Public Opinion Data" ( http://dsparks.wordpress.com/2011/10/24/
isarithmic-maps-of-public-opinion-data/ ), by David B Sparks
It isn't just on maps, of course, where properties of color can be important to
distinguish quantitative values. One of the most popular methods for coloring
involves the traffic light metaphor of red, amber, and green. This is commonly
used in corporate settings to indicate good, average, or bad performance thresholds.
However, it is important to know that around 10 percent of the population
(particularly males) has a red-green color deficiency. The use of an approach such
as the traffic light colors will therefore potentially alienate a significant proportion
of your intended audience. An effective alternative is to switch green for blue, so
positive values are now shown as blue and negative are still in red, as we see in the
following horizon chart: