Conceiving and Reasoning Visualization Design Options - Data Visualization: A Successful Design Process - page 96

There are seven distinct categories of data distinguished by a unique color. The color
itself has no meaning; it is purely a means of helping to separate out the various
tracks of issues. The lightness of the color does add an extra layer of information,
indicating where maximum (darker) and limited (lighter) rights are in place, and
the absence of any color as well as the presence of a cross-hatching pattern further
encodes extra meaning:
Image from "Gay rights in the US, state by state" ( http://www.guardian.co.uk/world/
interactive/2012/may/08/gay-rights-united-states ), by Guardian in America Interactive
As we saw in the earlier image showing the political persuasion map of the U.S.,
the use of color for categorical data also allows us to maximize the implication of
metaphorical or representative association.
However, regardless of whether the color depiction of categories is arbitrary or
embodies more meaningful association, one of the key rules we need to obey is that
the eye is only really capable of distinguishing up to a maximum of twelve different
color classifications. This is just one of the many fascinating aspects of color that can
be discovered from great topics such as Visual Thinking by Design , by Colin Ware.