HMI colour: what each one should mean, and how many you need

Colour on an operator screen is a budget. It is small, it is spent early, and most designs have spent all of it before they get to the thing that matters.

Spend it on abnormality

The rule that makes everything else follow: colour indicates that something needs attention. Nothing normal is coloured.

That leaves the whole palette available for priority, which is the one distinction an operator has to make instantly.

A palette that works

Three or four priorities, one visual treatment each, and consistent everywhere: banner, summary, the object on the mimic, and the faceplate.

• Critical. The most saturated treatment available, usually red, used for nothing else on the entire system.
• High. Amber or orange.
• Medium and low. Progressively less prominent, and honestly, often better shown by position in a list than by a colour of their own.

Unacknowledged needs to be distinguishable from acknowledged at a glance, and flashing is the traditional answer. Flashing is also genuinely unpleasant on a screen somebody sits in front of for twelve hours, so a border or a distinct fill is worth considering instead.

The red for running trap

Electrical panels have used red for energised and green for safe for a century, and it is a real convention with a real safety rationale on a panel front.

On a screen it collides with the alarm palette, and the collision is expensive: red now means both running normally and critical alarm, and the operator has to work out which from context. Under load they will not.

The usual resolution is to show run state with fill or shape, keep red for alarms only, and if the site convention genuinely cannot be moved, choose the alarm palette to avoid it.

Colour alone is never enough

Around eight percent of men have some form of colour vision deficiency, and red and green are the pair most often confused. On a shift team of a dozen, the odds are that somebody is affected.

The fix is not to avoid colour. It is to make sure colour is always redundant: a coloured state also has a shape, a symbol, a position or a word.

The test takes ten seconds. Screenshot the screen, convert it to greyscale, and see whether you can still tell what is wrong. If you cannot, neither can some of the people who run the plant.

The background

Light grey. Not white, which is fatiguing over a shift and leaves nowhere to go for a highlight. Not dark blue or black, which was a CRT era decision that survived on aesthetics and makes light text ghost on modern panels.

Where a dark theme is genuinely wanted, for a night shift control room, it needs to be designed as its own palette rather than an inversion of the light one, because inverting a palette moves every contrast relationship in it.

Consistency beats correctness

Last, and worth more than any individual choice: whatever palette is decided must be used identically on every screen, in every project, by every engineer.

An operator who has learned the palette can read a screen they have never seen. One who has learned that it varies by whoever built it has to read the legend every time, which means they will not.