Core Color-Coding Requirements
Apply one consistent, simple, and unambiguous color convention throughout every display in the facility and across the HMI system. Never make color the only carrier of process information. Reinforce each color state with text, a symbol, shape, size, intensity, or inverse video so the necessary information remains available on a monochromatic display or printout and to operators with color-vision impairment.
| Design decision | Requirement or guidance | Implementation |
|---|---|---|
| Color meaning | Requirement | Keep each meaning consistent across all displays. |
| State communication | Requirement | Pair color with a non-color display feature. |
| Red and green | Requirement | Add clarifying text next to the indicator. |
| Contrasting colors | Guidance | Select combinations for clear contrast; avoid red characters on a green background. |
Select a Minimal, High-Contrast Palette
Use only the colors needed to communicate sufficient information—usually no more than eight coding colors. Eliminate decorative color because it can obscure the operational meaning of the palette. Use highly saturated coding colors that contrast clearly with one another and with their backgrounds.
Treat any “Contrasts Well With” color pairing as a recommendation rather than a requirement. Evaluate the complete foreground-and-background combination instead of approving colors individually.
Encode Values and Immediate-Attention States
Do not represent relative process values through gradual changes in color intensity. Use another explicit display feature to convey the variable value or state. When an item requires immediate operator attention, such as an alarm, include a flashing or audible indication rather than relying on a static color change alone.
Review the HMI from a Human-Factors Perspective
During design, review process-control systems and components for correct application of both color and shape conventions. Verify that every state remains understandable without color, red and green indicators include adjacent explanatory text, meanings do not change between displays, and attention-critical items provide flashing or audible indication.
FAQ
How many colors should a process-control HMI use?
Keep the coding palette to the minimum needed to communicate sufficient information, usually no more than eight colors. Remove colors used only for decoration.
Can an HMI use red and green status indicators?
Yes, but place clarifying text next to each indicator and provide a redundant non-color cue. Avoid low-contrast combinations such as red characters on a green background.
Should an alarm rely on a color change alone?
No. When a display item requires immediate operator attention, include a flashing or audible indication, and retain a redundant feature that communicates the state without color.