Highlighting cannot rely on color alone
Aliases: non-color cue · color vision deficiency · state indication
What it is
Redundant coding expresses a highlighted, selected, erroneous, or prioritized state through at least one cue beyond hue—such as shape, outline, icon, text, position, or texture. Color is not prohibited; it simply cannot be the only evidence needed to understand the state. A highlight is carrying its information only if it remains distinguishable in grayscale, dim display conditions, projection, night mode, and for people with differing color vision.
Why it happens
Color perception varies with color vision, display calibration, ambient light, compression, and neighboring colors. Even people who distinguish two hues may not interpret them in a dense list or a quick scan. Non-color cues provide independent evidence: outlines establish object boundaries, icons and words supply semantics, and position or shape support peripheral recognition. Redundancy is not decorative accumulation; it makes a critical distinction recoverable when one signal weakens.
Studying it
Start by checking the interface in grayscale, at different brightness levels, and under common color-vision simulations, then run target-finding, state-judgment, and error-recovery tasks on real devices. Measure state-recognition accuracy, search time, and erroneous action for color-only and redundant versions. Simulation cannot replace participant testing: display quality, glare, fatigue, and divided attention also defeat a color-only cue. Verify that secondary cues survive zooming, responsive layouts, and localization.
Where it stops holding
Redundancy does not mean labeling every colored element. Decorative color, branding with no behavioral consequence, and imagery fully explained by adjacent text need not receive another cue. Nor does an arbitrary icon create useful redundancy without a stable mapping. Too many borders, badges, and labels damage hierarchy, so prioritize states that must be distinguished quickly and that change what an action will do.
Applying it
- Inventory color-coded states and their consequences; give consequential states a stable outline, symbol, text, or structural change.
- Perform real tasks in grayscale, low brightness, glare, and varied zoom—not just a color-value comparison in a mock-up.
- Keep each non-color cue consistently mapped to its state across comparable components.
- Test recognition and recovery: participants should explain the difference and the available action, not merely report that “the color changed.”