Use uniform perceptual spacing in continuous colormaps

Use a continuous colormap whose equal data intervals produce equal-looking color steps. For example, redistribute palette stops so equal-value intervals change by similar visible amounts instead of compressing one region and stretching another.

Uneven color spacing makes equal numeric differences look unequal. Some parts of the scale then appear over-emphasized while others lose visible separation.

Mechanism: Constant perceptual speed keeps equal data steps visually comparable, so readers do not overread fast regions or underread slow regions of the same scale.

Evidence: The collated review includes this paper as theoretical guidance for color-hue and color-saturation encoding, and the paper defines local and global uniformity as constant perceptual speed and evaluates departures from that rule through speed variation (Zeng & Battle, 2023; Bujack et al., 2018).

• User Goal: Compare equal-sized changes across an ordered color scale.
• Task: Read differences or intervals from multiple parts of the same palette.
• Data: Ordered values encoded on a continuous scale.
• Chart Setting: Static color legend or continuous color encoding where several parts of the scale must carry similar visual weight.
• Audience: Viewers interpreting magnitude from the relative size of visible color changes.
• Success Criterion: Equal numeric intervals look equally different across the full scale.

Break it when: The main priority is a different palette property, such as maximizing discriminative power rather than keeping equal steps visually equal. Why: The paper treats colormap rules as beneficial qualities that can trade off against each other.

Sacrifice: You may reduce emphasis in visually dramatic regions of the palette. Risk: Uniformizing the scale can lower separation where you previously had very large color jumps. Mitigation: Check both uniformity and minimum step size so equal spacing does not collapse local discrimination.

Mistake: Keeping equal numeric breaks while allowing one section of the palette to change much faster than another. Why it fails: Readers perceive some equal intervals as larger or smaller than others.

Failure Sign: Equal-value intervals produce visibly different jump sizes across the scale. Quick Check: Sample equal numeric intervals and compare whether the color jumps look similar. Stronger Test: Compute the standard deviation of local or global speed and reduce it toward zero.

• Move palette stops so equal-value intervals have similar perceptual distances.
• Remove compressed plateaus and overly steep jumps in the scale.
• Replace the scale with one whose local speed varies less across the full range.