Avoid a diverging colormap when readers must compare values across the midpoint

Use a sequential ramp instead of a diverging ramp when readers must judge which of two values is closer and the compared values can fall on opposite sides of the midpoint. For example, replace a blue-orange diverging scale with a sequential ramp for closeness comparisons that cross the center; the diverging scale only matched sequential behavior when all compared values stayed on one half.

A diverging ramp invites grouping by side of the midpoint, so a numerically closer neutral or achromatic value can lose to a farther chromatic value on the same side.

Mechanism: When values straddle the midpoint, readers can treat the two chromatic sides as separate groups and misread which option is actually nearest to the reference.

Evidence: The tested blue-orange diverging scheme showed elevated errors specifically when comparisons crossed the central hue boundary, while comparisons confined to one half behaved like the underlying single-hue sequential scales (Liu & Heer, 2018).

Notes: Diverging colormaps are still appropriate when the intended meaning is distance from a neutral midpoint.

• User Goal: Decide which value is closer or more similar.
• Task: Compare values that may lie on opposite sides of a central reference or zero point.
• Data: Ordered quantitative data encoded with a midpoint-centered color scale.
• Chart Setting: A continuous legend where readers must compare across both sides of the midpoint.
• Audience: Viewers reading relative distance directly from color.
• Success Criterion: Correct midpoint-crossing comparisons.

Break it when: The main message is distance from a neutral midpoint or readers mostly compare values within one side of the scale. Why: Diverging scales are designed for midpoint-centered interpretation, and the study found within-half comparisons behaved like the corresponding sequential halves.

Sacrifice: You lose the explicit visual emphasis of a neutral center. Risk: A sequential replacement can weaken the sense of deviation around a reference point. Mitigation: Keep the diverging scale only when midpoint meaning matters more than cross-midpoint closeness judgments.

Mistake: Assuming the midpoint-side option will still read as closest when its color is neutral and the farther option shares the reference hue family. Why it fails: Readers can group the chromatic colors together and miss that the achromatic option is numerically closer.

Failure Sign: Readers miss obvious closeness relationships when one comparison color falls on the opposite side of the midpoint. Quick Check: Test a few representative triplets that cross the midpoint and ask which option is closer to the reference. Stronger Test: Compare the same chart with the current diverging ramp and a sequential ramp on midpoint-crossing comparisons and keep the version with fewer errors.

• Replace the diverging colormap with a sequential colormap when cross-midpoint closeness is the main task.
• Keep the diverging colormap only if the chart is primarily about distance from the midpoint.
• Re-test representative midpoint-crossing comparisons after the palette change.