Recently, someone in the Mala GIS group shared an experience: a map they had worked hard to design was inexplicably rejected by the client, who kept saying the colors were wrong. After many revisions, it turned out the client had red-green color blindness. Upon checking, I learned that about 8% of males and 0.5% of females have some form of color vision deficiency, with red-green color blindness being the most common. So how do you create a map that works for people with color vision deficiency? This article shares some common principles and tools. Later, I will write another article introducing implementation methods in GIS software.

Color Design Principles

Color vision deficiency does not mean seeing no color; it means difficulty distinguishing certain hues, especially red and green when placed next to each other. In thematic maps, legends, and line symbols that rely solely on color coding, readers with red-green deficiency may perceive adjacent categories as the same class.

This issue has long been recognized. For example, Swiss cartographers Bernhard Jenny and Nathaniel Vaughn Kelso discussed it systematically in Color Design for the Color Vision Impaired, and the UK Ordnance Survey conducted early experiments on maps for users with color vision deficiency. The practical experience can be summarized as follows:

  • Increase lightness contrast. Even if adjacent categories differ in hue, if their lightness is too similar, users with color vision deficiency will struggle to tell them apart.
  • Limit the number of colors.
  • Prioritize proven color schemes, such as the colorblind-safe ramps on ColorBrewer.
  • Make line features appropriately thicker and add a white stroke to line symbols. This prevents intersecting lines from blending into a mess and makes overlapping red and green lines easier to recognize.
  • Distinguish categories using textures, line patterns, and point shapes, not just color.
  • Include color, line style, and text descriptions together in the legend.
  • Simplify map information where appropriate.
  • Avoid using the same color family for both the basemap and the thematic layers.

Common Color Tools

In addition to the principles above, we can use some mature tools to check our maps during the design process. Commonly used ones include:

ColorBrewer

URL: http://colorbrewer2.org/

Simply enable the "colorblind safe" option when selecting colors. It has long been integrated into mainstream tools like ArcGIS, R, and QGIS. For example, in QGIS:

Vischeck

URL: https://www.vischeck.com/

This tool is less convenient; it requires uploading a screenshot of the map to view online simulations for various types of color vision deficiency.

ColorHexa

URL: https://www.colorhexa.com/

Enter a color name, Hex, RGB, or HSL value. At the bottom of the page, there is a blindness simulator that shows how people with color vision deficiency perceive the color. If you already know the Hex value, you can also append #blindness-simulator to the URL to quickly open the simulation page, for example, https://www.colorhexa.com/00ff00#blindness-simulator.

Additionally, post-processing software like GIMP and Photoshop includes built-in color vision deficiency filters, which can be used for a final check after exporting the map. Since version 2.4, QGIS has had a built-in color vision deficiency simulation preview that can be toggled directly in the map composer interface, eliminating the need to export back and forth.

ESRI Mapping Center provides symbol style packs designed for color vision deficiency, which ArcGIS users can download and apply directly. ArcGIS Pro now not only includes scientifically designed colorblind-safe color schemes, but also has a professional color vision deficiency simulator tool built in, allowing users to preview and adjust seamlessly while mapping.

Conclusion

The essence of cartography is to convey information. Readers may include minorities such as those with color vision deficiency or color weakness. As GIS practitioners, I hope we can accommodate these audiences when possible. Moreover, catering to minorities does not mean making maps dull. Increasing lightness contrast, reducing the number of colors, adding textures and clear legend annotations, and then reviewing with tools like ColorBrewer and the QGIS preview only takes a few extra minutes. That little bit of time makes your map accessible and useful to more people.

Have you used similar color schemes in your mapping? Do you have better solutions? Leave a comment and share your thoughts.