Add patterns to map regions when confusing colors must remain
For color-coded maps, prefer texture encoding on filled regions to improve accessibility and mitigate confusing hue distinctions for readers with color-vision deficiency.
- purpose:refine
- basis:heuristic
- chart:map
- data:geospatial
- quality:accessibility
- lever:encoding
- channel:texture:use
- needs:color-vision-deficiency
advice
Layer pattern over region color
Add a pattern to a map region when adjacent areas would otherwise differ only by a hard-to-distinguish color pair. For example, keep the existing fills but overlay one region with a line pattern so neighboring countries or areas remain distinguishable even when the hues collapse.
reason
Why patterns help on maps
Filled geographic regions often depend on area color, and neighboring regions make confusion obvious. Pattern adds a second region identity that does not depend on hue alone.
Mechanism: Texture survives hue confusion and gives each filled area a visible difference even when the base colors look similar.
Evidence: The article shows a map that is unreadable to red-blind readers when adjacent regions differ only by color, then shows that adding a pattern solves the problem while keeping the same colors. (Muth, 2020)
context
Use when adjacent regions are color-only
- User Goal: Distinguish neighboring regions on a map.
- Task: Identify or compare geospatial categories.
- Data: Filled geographic areas differentiated by color.
- Chart Setting: A map where confusing colors must stay in place.
- Audience: Readers who may include people with color-vision deficiency.
- Success Criterion: Neighboring regions remain clearly separate without relying only on hue.
exceptions
When the pattern needs adjustment
Break it when: The added pattern makes a region look falsely lighter or darker than intended. Why: The article warns that patterns change how bright or dark the colors are perceived.
costs
Costs of adding patterns
Sacrifice: You give up a fully smooth fill appearance. Risk: The pattern can alter perceived lightness and distort the apparent color balance. Mitigation: Recheck the region fills after the pattern is added and adjust the base colors if needed.
mistakes
Common map-pattern failure
Mistake: Adding a pattern without checking how it changes the apparent brightness of the fill. Why it fails: The map may become distinguishable but still mislead readers about which region looks lighter or darker.
check
How to verify region patterns
Failure Sign: The patterned region now appears much lighter or darker than the unpatterned region for reasons unrelated to the data. Quick Check: Compare the map before and after the pattern to see whether the pattern has shifted perceived lightness. Stronger Test: Run a colorblind simulation and confirm that both the region identity and the apparent lightness still make sense.
fix
What to change
- Overlay one of the confusing regions with a clear pattern.
- Rebalance the base fill colors if the pattern changes perceived brightness too much.
- Keep the pattern only where it solves a real color-confusion problem.