Choosing colors for maps, charts, and infographics can be a difficult task. Standard software packages like Excel and Matlab have tended to offer substandard color schemes (though this is getting better) so it’s up to the designer to learn about color and apply the appropriate techniques.
This post will focus on color schemes for maps. There are several types to consider:
- Sequential – good for quantitative variables.
- Divergent – good for quantitative variables with an obvious midpoint (e.g. zero) and two divergent tails.
- Qualitative – good for categorical variables.
Given a sequential, non-divergent color scheme, Cynthia Brewer and Mark Harrower (of ColorBrewer fame) note:
Lightness steps dominate the look of these schemes, usually with light colours for low data values and dark colours for high values. ‘Dark equals more’ is a standard cartographic convention. Sequential schemes can be either single hue (e.g. same blue, with different lightness and saturation levels) or multi-hued (e.g. light yellow through dark green).
It’s important first to remember that computer displays and common digital color formats (like RGB) only show a subset of all visible color. It’s also good to be aware of some common color spaces: RGB, HSV, and YIQ.
RGB identifies color by red, green, and blue components. HSV uses hue (angle on the color wheel), saturation (intensity of color), and value (amount of black). YIQ is used for NTSC television signals. It has a black/white component plus two chromatic components; the black/white Y component is what is visible on a black and white television.
Much experimentation has been done on how people perceive colors. Assuming colors are at equal saturation. “Value perception dominates color perception.” (Bertin, p 87) Images generated earlier on this blog tested color sorting across every variation of RGB, HSV, YIQ color spaces and confirm Bertin’s assessment.
Bertin writes, “The saturated tone is not of constant value but varies in value according to the hue.” This makes sense. For example, yellow at 100% saturation is much brighter than blue at 100% saturation. If yellow and blue are to be used in the same color scheme, we will need to compensate for the perceived brightness of the yellow by decreasing its value.
Munsell Color System
Enter Albert H. Munsell. In the early 1900′s he undertook a lot of research to understand exactly how it is people perceive color. The resulting Munsell Color System is a three-axis system that doesn’t conform to any standard spherical/cubic geometric model.
The central axis indicates the perceived value (darkness/lightness) of the color. The angular values indicate hue, and length out from the axis is chroma (intensity).
When choosing colors for maps and other diagrams that utilize a linear, quantitative scale, the Munsell color system would be a good place to start. ColorBrewer has great color schemes for maps, and all of them have proven perceptual merit. ColorBrewer is a great resource and I have used it in many projects. The only real issue with ColorBrewer is that it contains so few color schemes. Sometimes a designer wants to be able to match the brand, identity, or theme of a larger project while maintaining the integrity of the diagram’s visual communication. Working with the Munsell system should allow just that.
We have not tested this yet, but one could choose a particular hue + chroma in the Munsell system as a starting point, and work up and down the axis to find related colors that vary on perceived value. Additionally, one could move in a diagonal fashion, moving up in value while also moving an angular increment around the axis. This would theoretically accomplish the multi-hued sequential schemes that Harrower and Brewer describe in their paper on ColorBrewer (quoted above).
Jacques Bertin, Semiology of Graphics. Translated 1983 by William J. Berg, originally published in 1967.
Mark Harrower and Cynthia Brewer, Colorbrewer.org: An Online Tool for Selecting Colour Schemes for Maps. The Cartographic Journal, Vol 40, No 1, pp 27-37. June 2003. [PDF]
Bruce MacEvoy, Modern Color Models. 2005.