How to Design Colorblind-Friendly HMI Palettes for Industrial Control

flowchart LR
    A["Operator Input"] -->|Color Deficient| B{"Process State"}
    B -->|Green Hue| C["Perceived as Gray"]
    B -->|Red Hue| D["Perceived as Gray"]
    C --> E["Safety Risk"]
    D --> E
    style A fill:#1e3a8a,stroke:#0f172a,color:#ffffff
    style B fill:#3b82f6,stroke:#0f172a,color:#ffffff
    style C fill:#94a3b8,stroke:#0f172a,color:#000000
    style D fill:#94a3b8,stroke:#0f172a,color:#000000
    style E fill:#ef4444,stroke:#7f1d1d,color:#ffffff

Red means stop, green means go. It is a universal rule embedded in the minds of almost every industrial automation engineer. However, for approximately 8% of men and 0.5% of women operating machinery on the plant floor, distinguishing between a green “Running” indicator and a red “Fault” alarm is a daily struggle. Relying solely on color to convey machine states is a severe safety hazard that can lead to catastrophic misinterpretations. Designing a colorblind-friendly HMI is not just an accessibility tick-box; it is a fundamental requirement for robust industrial operations.

The Risk of Red-Green Color Vision Deficiency

The most common form of color vision deficiency (CVD) is Protanopia and Deuteranopia, commonly referred to as red-green colorblindness. In high-stakes environments like chemical processing or heavy manufacturing, an operator who cannot differentiate between a normal operational state (green) and an alarm state (red) might acknowledge an alarm incorrectly or fail to stop a sequence in time. The ISA-101 standard for Human Machine Interfaces emphasizes the use of high-performance graphics, which inherently supports accessibility by muting background colors and using bright colors only for abnormal situations.

flowchart TD
    State["Machine State"] -->|Normal| Gray["Gray / Muted Blue Indicator"]
    State -->|Warning| Yellow["Yellow Indicator + Triangle Shape"]
    State -->|Alarm| Red["Red Indicator + Diamond Shape + Flashing"]
    style State fill:#2e5c8a,stroke:#1a365d,color:#fff
    style Gray fill:#808080,stroke:#333,color:#fff
    style Yellow fill:#e6b800,stroke:#333,color:#000
    style Red fill:#cc0000,stroke:#333,color:#fff

Strategies for a Colorblind-Friendly HMI

Creating an accessible SCADA or HMI screen requires a paradigm shift from traditional design. The focus must move from color coding to redundant coding. Redundant coding means conveying information through at least two different visual channels simultaneously.

1. Implementing Shape-Based State Indication

Never rely on color alone. If a pump turns red when faulted, it must also change shape or display a text label. A common convention is to use a specific geometric shape for each state.

• Normal (Running): Circle.
• Warning (Unacknowledged): Triangle with an exclamation mark.
• Fault (Critical): Diamond or square with an “X”.
• Stopped (Off): Simple outline or grayed-out shape.

2. Leveraging Contrast and Luminance

Colorblind operators often distinguish colors by their luminance (brightness) rather than their hue. If you convert your HMI screen to grayscale, the difference between a running state and a stopped state should still be immediately obvious. A dark red and a dark green will look identical in grayscale. Ensure your fault colors have a significantly different brightness level than your normal operating colors. Many modern graphic libraries provide palettes specifically tested for adequate luminance contrast.

3. Utilizing Text and Symbology

Text is unambiguous. Whenever space permits, overlay the state text directly on or next to the equipment graphic. Instead of just showing a colored circle next to a motor, add text like “RUN” or “FLT”. This eliminates all guesswork. Standardizing these abbreviations across the entire plant ensures that temporary operators or contractors can immediately understand the system state without needing specialized training on your specific color conventions.

The Role of Animation in Redundant Coding

Animation provides a powerful, color-independent method for drawing attention to critical events. A flashing indicator is universally recognized as an active, unacknowledged alarm, regardless of the observer’s color vision capabilities. However, animation must be used strategically. A screen with dozens of constantly spinning fans or flowing pipes creates visual noise that masks genuine alarms. Reserve animation exclusively for abnormal states that require immediate operator intervention.

Comparison of Traditional vs High-Performance Accessibility

Testing Your HMI Screens for Accessibility

Before deploying a new HMI project, it is critical to validate its accessibility. You do not need specialized hardware for this. Most modern operating systems include accessibility settings that allow you to simulate various forms of colorblindness on your engineering workstation. Alternatively, taking a simple screenshot and converting it to grayscale in any image editor is a highly effective way to identify areas where your design relies too heavily on color contrast instead of luminance or shape differences.

Overcoming Plant Floor Resistance

When migrating to a colorblind-friendly HMI layout, operators accustomed to the old bright screens will initially resist the change. They often complain that the new screens look boring or dead. This requires a calculated transition plan. Begin by involving operators in the design phase and demonstrate how quickly they can spot an alarm on a gray background compared to a screen cluttered with bright colors. Pilot the new screens on a secondary, less critical system before rolling it out plant-wide.

Conclusion

Implementing a colorblind-friendly HMI is a fundamental step in modernizing your control systems. By prioritizing shapes, contrast, and text over pure hue, you create a safer, more efficient operating environment that accommodates all personnel. Designing for accessibility ultimately leads to cleaner, more professional interfaces that benefit everyone, not just those with color vision deficiency. For more advanced components to build accessible interfaces, explore the SCADA templates available in our shop.

FAQ

Does ISA-101 mandate colorblind-friendly design?

While ISA-101 does not strictly use the term “colorblind,” its core principles of high-performance graphics (using muted backgrounds and redundant coding for alarms) inherently solve the main issues faced by operators with color vision deficiency.

Can I still use red and green?

Yes, but they must not be the only way a state is communicated. If you use red for a fault, you must also include a shape change, an icon, or explicit text indicating the fault.