Rule of Meaningful Color

Requirement

Choose color encodings according to what they represent: unordered categories, ordered magnitude, or deviation around a meaningful reference. Identify the mapping and preserve essential distinctions without color alone.

Why

ColorBrewer's scheme guidance distinguishes qualitative, sequential, and diverging encodings. This rule asks whether the mapping is appropriate; DR-005 asks whether an established meaning remains consistent. Those are separate questions.

Application

Use qualitative colors for carrier identity, a sequential treatment for increasing dwell, and a diverging treatment for deviations around an actual service target. Provide readable labels and a legend or equivalent explanation.

Exception

An explicitly ordered category set or a meaningful threshold may justify a different treatment. Explain it. A diverging midpoint must not imply an analytical reference merely because it is the mathematical midpoint of a palette.

Review

Inspect the variable's semantics and the actual renderer mapping. Matching color attributes cannot establish either. A grayscale or accessible data view can be a valid alternative when color adds no necessary distinction. Mark visibility: Meaningful marks must also be distinguishable against their actual map/chart substrate. A semantically appropriate palette can still have weak contrast. W3C non-text contrast guidance describes 3:1 against adjacent colors for relevant graphical objects. mark-contrast provides a partial check for solid CSS backgrounds only; complex paint is unassessed. Retain redundant labels and review the full rendered substrate. This does not automate semantic palette selection or certify accessibility. Expanded decision-surface reference.

Enforcement

review is the strongest mechanism Viewrule expects for the complete rule. Partial checks can still provide supporting evidence without proving the whole requirement.

Detection and enforcement details →