3 Color scales
There are three fundamental use cases for color in data visualizations: (i) we can use color to distinguish groups of data from each other; (ii) we can use color to represent data values; and (iii) we can use color to highlight. The types of colors we use and the way in which we use them are quite different for these three cases.
3.1 Color as a tool to distinguish
We frequently use color as a means to distinguish discrete items or groups that do not have an intrinsic order, such as different countries on a map or different manufacturers of a certain product. In this case, we use a qualitative color scale. Such a scale contains a finite set of specific colors that are chosen to look clearly distinct from each other while also being equivalent to each other. The second condition requires that no one color should stand out relative to the others. And, the colors should not create the impression of an order, as would be the case with a sequence of colors that get successively lighter. Such colors would create an apparent order among the items being colored, which by definition have no order.
Many appropriate qualitative color scales are readily available. Figure 3.1 shows three representative examples. In particular, the ColorBrewer project provides a nice selection of qualitative color scales, including both fairly light and fairly dark colors (Brewer 2017).
As an example of how we use qualitative color scales, consider Figure 3.2. It shows the percent population growth from 2000 to 2010 in U.S. states. I have arranged the states in order of their population growth, and I have colored them by geographic region. This coloring highlights that states in the same regions have experienced similar population growth. In particular, states in the West and the South have seen the largest population increases whereas states in the Midwest and the Northeast have grown much less.
3.2 Color to represent data values
Color can also be used to represent data values, such as income, temperature, or speed. In this case, we use a sequential color scale. Such a scale contains a sequence of colors that clearly indicate (i) which values are larger or smaller than which other ones and (ii) how distant two specific values are from each other. The second point implies that the color scale needs to vary uniformly across its entire range.
Sequential scales can be based on a single hue (e.g., from dark blue to light blue) or on multiple hues (e.g., from dark red to light yellow) (Figure 3.3). Multi-hue scales tend to follow color gradients that can be seen in the natural world, such as dark red, green, or blue to light yellow, or dark purple to light green. The reverse, e.g. dark yellow to light blue, looks unnatural and doesn’t make a useful sequential scale.
Representing data values as colors is particularly useful when we want to show how the data values vary across geographic regions. In this case, we can draw a map of the geographic regions and color them by the data values. Such maps are called choropleths. Figure 3.4 shows an example where I have mapped annual median income within each county in Texas onto a map of those counties.
In some cases, we need to visualize the deviation of data values in one of two directions relative to a neutral midpoint. One straightforward example is a dataset containing both positive and negative numbers. We may want to show those with different colors, so that it is immediately obvious whether a value is positive or negative as well as how far in either direction it deviates from zero. The appropriate color scale in this situation is a diverging color scale. We can think of a diverging scale as two sequential scales stiched together at a common midpoint, which usually is represented by a light color (Figure 3.5). Diverging scales need to be balanced, so that the progression from light colors in the center to dark colors on the outside is approximately the same in either direction. Otherwise, the perceived magnitude of a data value would depend on whether it fell above or below the midpoint value.
As an example application of a diverging color scale, consider Figure 3.6, which shows the percentage of people identifying as white in Texas counties. Even though percentage is always a positive number, a diverging scale is justified here, because 50% is a meaningful midpoint value. Numbers above 50% indicate that whites are in the majority and numbers below 50% indicate the opposite. The visualization clearly shows in which counties whites are in the majority, in which they are in the minority, and in which whites and non-whites occur in approximately equal proportions.
3.3 Color as a tool to highlight
Color can also be an effective tool to highlight specific elements in the data. There may be specific categories or values in the dataset that carry key information about the story we want to tell, and we can strengthen the story by emphasizing the relevant figure elements to the reader. An easy way to achieve this emphasis is to color these figure elements in a color or set of colors that vividly stand out against the rest of the figure. This effect can be achieved with accent color scales, which are color scales that contain both a set of subdued colors and a matching set of stronger, darker, and/or more saturated colors (Figure 3.7).
As an example of how the same data can support differing stories with different coloring approaches, I have created a variant of Figure 3.2 where now I highlight two specific states, Texas and Louisiana (Figure 3.8). Both states are in the South, they are immediate neighbors, and yet one state (Texas) was the fifth-fastest growing state within the U.S. whereas the other was the third slowest growing from 2000 to 2010.
When working with accent colors, it is critical that the baseline colors do not compete for attention. Notice how drab the baseline colors are in (Figure 3.8). Yet they work well to support the accent color. It is easy to make the mistake of using baseline colors that are too colorful, so that they end up competing for the reader’s attention against the accent colors. There is an easy remedy, however. Just remove all color from all elements in the figure except the highlighted data categories or points. An example of this strategy is provided in Figure 3.9.