The short answer
Color in HMI Design
On a well-designed HMI, color is information, not decoration: the background and the normal process depiction are grays, so that the few colors on the screen are the things that need attention. Alarm colors are assigned by priority in the alarm philosophy and used for nothing else, equipment running and stopped states are shown by fill contrast and a text label rather than green and red, bad quality and manual mode have their own distinct indications, and every color choice is paired with a shape, a text, or a position cue so that operators who cannot distinguish the colors still get the message. All of it is written in a style guide with a palette table that every display follows.
Key points
- Gray for normal. If everything is colored, nothing stands out.
- Alarm colors by priority, from the philosophy, used only for alarms.
- Running and stopped by fill and text, not green and red; red is an alarm color.
- Every color has a second cue: a shape, a label, a border, a position. Color-blind operators exist in every utility.
- One palette table in the style guide; the display that deviates is wrong, however pretty.
- Consistency across displays, sites, and years matters more than any single choice.
Why gray
The purpose of a control display is to let an operator see what is abnormal quickly. A display that draws every pipe in blue, every running pump in bright green, every tank in cyan, and every heading in red has spent its colors on things that are normal, and the one thing that is wrong is one colored object among a hundred. A display drawn in grays, with process lines in mid gray, equipment outlines in dark gray, and text in near black, is calm when the plant is calm. An alarm colored red or yellow on that display is the only saturated color on the screen and the eye goes to it without searching. That is the reasoning behind the high performance HMI palette, and it is the single most effective change a utility can make to its displays.
A palette
| Element | Color | Second cue |
|---|---|---|
| Display background | Light or medium gray | None needed |
| Process lines and vessels | Mid gray, thin | Line weight for the main flow path |
| Text and values | Near black | Font size by importance |
| Equipment running | Light fill, or white, with the outline | Text: RUN, and a state label on the faceplate |
| Equipment stopped | Dark gray fill | Text: STOP or OFF |
| Equipment in manual or local | A distinct marker, often a small tag or border in a reserved color | Text: HAND, LOCAL, MAN |
| Highest priority alarm | Red | A symbol such as a filled triangle, and the priority number |
| High priority alarm | Orange or amber | A different symbol, and the priority number |
| Medium priority alarm | Yellow | A different symbol |
| Low priority or advisory | Blue or cyan, muted | A different symbol |
| Bad quality or communication lost | Magenta, or a hatched fill, with the value replaced by a marker | The value itself shows a symbol, never a stale number |
| Interlocked or permissive blocked | A reserved marker color on the device | A symbol and a faceplate entry |
| Selected or focused item | A thin highlight border | Border only |
Running and stopped
Green for running and red for stopped is the convention from panel pilot lights, and it fails on a display for two reasons. Red is the color that must mean the highest alarm, and a screen full of stopped pumps in red is a screen full of false alarms. And green and red are the two colors that the most common color vision deficiency cannot tell apart. On the display, a running device is a light or white fill and a stopped one a dark fill, with a text label on or beside the symbol; the contrast is visible to everyone and leaves the alarm colors alone. A plant that is not ready to give up its pilot light colors can at least keep red off the running and stopped states and use the alarm colors only for alarms.
Color vision
Roughly one man in twelve has a color vision deficiency, most commonly a weakness in distinguishing red from green, and a utility with a few dozen operators has some. A display that conveys state only by color is unreadable to them, and they will not always say so. The rule is that every color carries a second cue: an alarm has a shape and a number as well as a color, a state has a text label as well as a fill, a bad value has a symbol as well as magenta. Testing a display in a grayscale rendering, or with a color-blindness simulator, shows whether the cues are enough.
Consistency
Whatever the palette, it is applied the same way on every display, at every site, by every integrator, for the life of the system. A pump that is drawn one way in the plant and another way at the lift stations, or an alarm color that means high at one site and medium at another, makes the operator translate, and translation under stress is where errors happen. The style guide holds the palette table, the symbol library, and the rules, and it is a deliverable that every project uses. A system with three generations of displays in three styles is due for a style guide and a migration.
Frequently asked questions
- Operators like the colors. Why change?
- Because the purpose of the display is to make the abnormal obvious, and the evidence from process industries is that gray displays with reserved alarm colors let operators detect and respond to abnormal conditions faster and with fewer errors. The preference for color fades after a few shifts with a calm display; the first time an operator spots a problem across the room, the argument is over.
- Which colors go with which priority?
- The alarm philosophy decides, and the platform implements it. A common assignment is red for the highest priority, orange or amber for high, yellow for medium, and blue or cyan for low. The colors must not be used for anything else on the display, and each must come with a distinct symbol.
- How do I show a value that is bad quality?
- Replace the number with a marker such as dashes or a question mark, color the field magenta or hatch it, and show the same marker on any trend or bar that uses the value. A stale number in a normal color is the worst outcome, because the operator will believe it.
- What about the colors on the physical panel?
- Panel pilot lights follow their own conventions and standards, with green often meaning running and red meaning stopped or fault depending on the site standard. The HMI does not need to match the panel; it needs to be consistent with itself. Where an operator uses both, a note in the style guide about the difference is worth writing.
Related topics
- High Performance HMI DesignGrey backgrounds, reserved color, and displays designed to make abnormal conditions obvious rather than to look impressive in a demo.
- ISA-101: Human Machine InterfacesWhat the ISA-101 standard for human machine interfaces covers, its lifecycle from philosophy to maintenance, the display hierarchy, and what adopting it changes on a project.
- Alarm Indication on the HMIHow an alarm looks and sounds on the operator screen: the ISA-101 and ISA-18.2 rules for color, shape, and blink, the alarm banner and summary, acknowledgment behavior, navigation to the alarm, and indication that survives colorblindness and a noisy control room.
- HMI FaceplatesThe pop-up that controls one device: what a pump, valve, or loop faceplate must show, the mode and command layout, interlock and permissive display, security by role, and building faceplates from a standard object so every device behaves the same.
- Alarm PriorityHow to assign alarm priorities that operators trust: a consequence-and-time matrix, three or four levels, the target distribution from ISA-18.2 and EEMUA 191, and the mistakes that make every alarm high.
- Situational AwarenessDesigning displays so an operator knows what is happening, what it means, and what comes next: the three levels of awareness, the display hierarchy, analog indicators with normal ranges, embedded trends, time-to-limit predictions, and a test for any display.
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