As you may recall from the Symbolization and Color Spaces Lesson 2 concept gallery items, there are three components of color that cartographers have to work with: hue, value and chroma. In this part of the lesson, we will discuss the different ways that you can use these three components to create different types of color schemes.
The main thing to remember when designing a color scheme is that you want the logic of your colors to relate to the logic in your data (i.e., if you are representing differences in the kind of things on your map, use the component of color that works best for showing nominal differences (hue)). We will discuss four main types of color schemes: sequential, diverging, qualitative and binary.
A sequential scheme is typically used to represent differences in the amount of the phenomenon you are mapping. This difference may be quantitative (e.g., inches of rainfall, hours of sunlight, etc.) or ordinal (e.g., least polluted to most polluted; least desirable vacation spot to most desirable vacation spot). Typically, we use color value combined with color chroma differences when we are creating sequential schemes (see Figure 4.cg.13 below). Experiments with map readers have shown that most map readers associate darker symbols with a larger quantity, and lighter symbols with a smaller quantity (McGranahan 1989), so this is a convention that cartographers generally use when designing a sequential scheme. Generally, map readers will not be able to tell the difference between more than six or seven levels of color value, especially in the complicated context of the map itself. It is possible to extend your sequence by using more than one hue in combination with value (e.g. from yellow through green to blue). This combination will allow you to create a larger number of symbols (that are still differentiable from each other) than you could with color value alone. A final consideration when creating your sequential schemes is that cartographers typically try to use value differences that are perceptually equal throughout the symbol set (i.e., we do not want the difference in lightness between any two neighboring symbols in the scheme to seem larger than the difference between other neighboring pairs).
A diverging scheme can be constructed by fusing two sequential schemes together, using a common color (typically white or another light color such as yellow or light gray) as the midpoint. Hence the name diverging, as this scheme is composed of two sequential schemes that diverge from a common color. Diverging schemes are most useful for making comparisons with some critical value in the data. You can choose to use any number of values as the critical value, ranging from zero (e.g., in a map of population change zero represents no change, with either side of the diverging sequence representing positive or negative population growth) to the mean or median (e.g., in a map of mortality from vehicle accidents (see Figure 4.cg.14,below) to highlight areas that are at higher or lower risk) to some targeted level (e.g., in a map of greenhouse gas emission reductions to emphasize how much more some countries have reduced their emissions beyond the target specified in a treaty and which countries have not met that target and how far they still have to go to meet the target). One research group has also found that diverging schemes have been better able to help map readers identify true clusters of high or low values on maps (and avoid seeing spurious ones), perhaps because of the added differentiation that a second hue brings to the map (Brewer et al. 1997).
A qualitative scheme mainly uses differences in color hue to indicate differences in the kind of some phenomenon (e.g., land use, crop type, religion, etc.). In a qualitative scheme, you will generally want to choose color hues that have approximately the same lightness and chroma level (see Figure 4.cg.15, below). Otherwise, you will find that more saturated or lighter colors really pop out from the map. One exception to this may be in cases where you have groups of related variables within the map. For example, if you were creating a map of foreign-born residents, but you also wanted to make a distinction between levels of residential segregation of new immigrants, you might choose to use a different hue for each continent of origin, and then specify two levels of that hue for each continent based on the proportion of the enumeration unit that group made up (e.g., if the county was more than 30% persons who were born in South America, it might be represented by a dark blue color, while a county where less than 30% of its foreign born residents came from South America would be represented with a lighter blue color).
Binary color schemes are a special case of qualitative or sequential color schemes that have only two categories. Depending on what you are aiming to represent, you may choose to use either color hue or color value for creating a binary scheme. For example, a map that depicted the candidate that most people voted for in the last presidential election might use color hue (e.g., blue and red are colors traditionally used in the United States for this type of map). In other cases, you might choose to use color value (e.g., if you are representing which locations are visible from a particular viewpoint, you might use black for areas that are not visible and white for areas that are visible).
If you are interested in investigating this subject further, I recommend the following:
- Brewer, C. 2013. ColorBrewer 2.0: Color Advice for Cartography. ColorBrewer2.org
- Brewer, C. 2015. Designing Better Maps: A Guide for GIS Users, 2nd Edition. ESRI Press.