Use a perceptually uniform multi-hue colormap for fine-grained quantitative comparison

Use a perceptually uniform multi-hue colormap when readers must distinguish nearby values on a continuous quantitative scale. For example, replace a single-hue blues ramp with viridis on a heatmap or scalar field when small span differences matter; viridis stayed accurate where single-hue ramps lost resolution.

A multi-hue ramp can separate nearby values more clearly than a single-hue ramp because hue and luminance both change while the scale still preserves order.

Mechanism: Adding hue variation to a luminance ramp increases separation between nearby values, so readers make fewer mistakes when judging which value is closest to a reference.

Evidence: Viridis was among the fastest and most accurate tested colormaps, and single-hue ramps showed a clear error increase at the smallest spans where nearby values had to be distinguished; across studies, the perceptually uniform multi-hue ramps had the lowest error overall (Liu & Heer, 2018).

Notes: Single-hue ramps remained competitive when value differences were larger.

• User Goal: Compare which values are closest or most similar.
• Task: Judge relative distance between nearby values on an ordered color scale.
• Data: Ordered quantitative values encoded by a continuous color scale.
• Chart Setting: A scalar field or other view with a continuous legend where local differences matter.
• Audience: Viewers reading the color scale directly.
• Success Criterion: Low error on near-value comparisons without slowing readers.

Break it when: The scale is discretized to only a few bins and the task does not depend on distinguishing very small value differences. Why: The paper notes that single-hue colormaps may still be acceptable for discrete scales with about 5–7 colors, and the tested single-hue ramps performed well at larger spans.

Sacrifice: You give up the visual simplicity of a single-hue ramp. Risk: A multi-hue ramp that is not judiciously designed can still read poorly. Mitigation: Use a perceptually uniform multi-hue ramp that also ramps in luminance, as in viridis.

Mistake: Keeping a single-hue ramp after increasing the number of bins or asking readers to separate nearby values. Why it fails: The tested single-hue ramps showed a strong resolution drop at the smallest spans.

Failure Sign: Adjacent colors look different enough, but deciding which of two nearby values is closer to a reference still feels uncertain. Quick Check: Sample three nearby values from the scale and ask which comparison color is closer to the reference. Stronger Test: Compare the same chart with the current single-hue ramp and a perceptually uniform multi-hue ramp on low-span comparisons and keep the version with fewer errors.

• Replace the single-hue ramp with a perceptually uniform multi-hue ramp such as viridis.
• Preserve a luminance ramp while adding hue change across the scale.
• If you must keep a discrete scale, keep the number of bins small enough that readers are not forced into low-span judgments.