Colour, contrast and the control room environment

Display colour is usually chosen by whoever built the graphics, on their own monitor, in daylight, in a meeting room. It is used on a console screen, under control room lighting, at three in the morning, by an operator who may have colour vision deficiency and will certainly have degraded attention.

The gap between those two conditions accounts for a substantial share of display usability problems.

A colour scheme is a semantic system

The useful way to think about colour is as a language with a limited vocabulary. Every colour used must mean something, that meaning must be consistent across every display, and the vocabulary must be small enough to be learned and remembered.

Online timesheets can provide a lightweight record of effort across maintenance, commissioning and support tasks. Further details are available in this overview.

Additional industry context is available in the ISA-101 HMI standards.

Traditional schemes fail this because colour carries several meanings simultaneously: pipe contents, equipment type, running state, and alarm condition, all encoded in hue. An operator seeing a red item has to determine which of four possible meanings applies.

The high-performance approach reduces the vocabulary drastically: colour means abnormality and priority, and nothing else. Everything else is encoded in shape, label and position.

Colour vision deficiency is common

Around one in twelve men has some form of colour vision deficiency, most commonly affecting red-green discrimination. In a control room population of thirty operators, this is not an edge case.

The practical consequence is that colour must never be the only channel carrying critical information. Alarm priority should also be distinguished by position in a list, by shape or by an accompanying symbol. A scheme where high priority is red and medium is amber, with no other difference, is unreadable by a meaningful proportion of the people using it.

Testing is straightforward: view the display through a colour vision deficiency simulation, or simply in greyscale. If priority is still distinguishable, the design is robust.

Contrast and legibility

Text on process displays is frequently small and low in contrast, because the design was evaluated on a large monitor at close range.

The operational reality is a screen viewed from a metre or more, sometimes at an angle, by someone who may need reading glasses. Text should be larger than feels necessary, and contrast between text and background should be generous.

The mid-grey background helps here: dark grey text on mid grey provides adequate contrast without the harshness of black on white, and leaves room for genuinely high-contrast elements to stand out.

The environment matters

Control room lighting, screen glare, viewing angle and screen age all affect what the operator actually sees.

An old console monitor with degraded backlight renders a subtle colour scheme as mush. A screen positioned opposite a window has usable contrast for part of the day. These are not display design problems and they defeat display design, which is why an assessment of the physical environment belongs in any display project.

Blinking and animation

Blinking is a powerful attention mechanism and is routinely overused. A display with several blinking elements has no attention mechanism, because everything is competing.

The defensible convention is that blinking indicates unacknowledged alarm and stops on acknowledgement. Nothing else blinks, ever. This keeps the mechanism meaningful.

Animation of process flow — moving dashes along pipes, rotating pump symbols — is decorative, consumes attention, and provides information that is available more reliably from a running indication. It is one of the first things to remove.

Depth, gradients and realism

Three-dimensional rendering, gradients, drop shadows and photorealistic equipment images were the mark of a sophisticated graphics package twenty years ago and persist for that reason.

They cost attention and screen space, they reduce the contrast available for real indications, and they carry no operational information. A vessel drawn as a plain outline conveys the same thing as one rendered with lighting effects, in a fraction of the visual budget.

Making the transition

Moving an established control room from a traditional to a high-performance colour scheme is a change management exercise as much as an engineering one.

Operators have years of trained response tied to the existing colours, and the transition period is genuinely uncomfortable. Doing it one display at a time creates a period where two conventions are in use simultaneously, which is worse.

The approaches that work are either a complete changeover for a console at a planned point, with training beforehand and support during, or a parallel period where the new displays are available alongside the old ones and operators migrate as they become comfortable.

What does not work is imposing the change without explanation. Operators who understand why colour is being reserved for abnormality accept it quickly. Operators who experience it as their screens being made grey by someone in an office do not.

Testing the scheme where it will be used

A colour scheme should be evaluated on the actual console hardware, under actual control room lighting, at the actual viewing distance, at several times of day.

Screens differ substantially in colour rendering and brightness, particularly as they age. A palette that is clearly differentiated on a new monitor may lose distinction on a six-year-old console screen with a degraded backlight.

The practical test is to display the palette on every console position and confirm that each colour remains distinguishable. Where it does not, the answer is either a wider palette separation or a screen replacement programme, and knowing which is useful.

Night operation

Control rooms are typically lit more dimly at night, and operators on a night shift have reduced attention and different visual adaptation.

Some sites provide a dimmed display scheme for night use. This helps with glare and introduces a risk: a scheme with reduced contrast may make abnormality indications less conspicuous exactly when the operator is least alert.

Where a night scheme is used, it should reduce background brightness while preserving the contrast of alarm indications rather than dimming everything uniformly.

Consistency with the field

Colour conventions used on the display should not contradict those used physically on the plant, in local panels, or in printed procedures.

Where the site standard for a running motor is green on a local panel and the display scheme shows running equipment in grey, operators are working with two conventions. This is manageable and worth being deliberate about rather than discovering it during commissioning.

Documenting the palette as values

A colour scheme described in words — dark grey background, amber for medium priority — will be implemented differently by each person who reads it.

The palette should be specified as exact values, held in the style guide and, where the platform permits, defined once in the graphics system so that displays reference the definition rather than repeating literal colours.

The practical benefit appears when the scheme is adjusted: a palette defined centrally can be changed once, while one embedded in four hundred displays cannot be changed at all.