The Understated Importance of the Cartographic Element of Color

Color, inasmuch as it is used as a cartographic tool, is probably taken for granted by most map viewers- another map element unremarkable the vast majority of the time, except when it is used poorly, and then becomes unmissable.  The color palette chosen by the map maker isn't purely an aesthetic decision either, it is carefully considered, so that the feature being displayed appears in a logical context.  A map with purple water features and blue forest tracts wouldn't be considered seriously by most, save as a piece of art.  Color is also of utmost importance within the context of the choropleth map, which displays some feature, grouped into classed categories, based upon some measurement.  Color variation for different classes is often seen as a change in saturation or lightness, within the same general hue.

Above is a linear color ramp- the darkest and lightest shades vary by a certain amount for the R, G and B values, and the classes/shades between are a linear progression of those values, yet the color remains a constant hue.  

The light/dark progression of one hue is, however, augmented, as in the ramp above, by adjusting the amount of the interval between the R, G and B values between the darkest colors.  The object here, were these colors being used in a choropleth map, would be to retain the same hue for all of the classes, while appreciably varying the lightness and saturation, so that whatever is being symbolized on the map is shaded in colors different enough to tell one apart from the other, based on the value of the hue.

Finally, we have a color ramp produced by Colorbrewer.org, which is an exceedingly useful tool for generating color ramps such as these.  The above ramp uses a slightly different hue for the top, darkest class, and then varies the lightness of the pinker hue in the classes below.  The use of this type of ramp would be invaluable in a map with which one might want to display different areas in different shades, to indicate the amount of some measurable thing- the colors are all in approximately the same hue, and yet vary enough from one another such that one area would be visually distinct from another.