How many colors an HMI screen palette really shows

What a palette count documents

A palette capacity is the number of color or gray entries the project software can assign to an object, and it behaves like a maximum tag count: a limit the editor enforces. The count tells you nothing about which hues fill the slots, because each software package carries its own palette definition. Two projects can accept the same entry count and still render different colors at the same index, so open the palette in the target project and compare entries before borrowing colors between projects.

Both rows are examples of documented capacities. Your panel's own documentation prints its count, and that count is the ceiling for assignments, never the design target.

About 20 entries is the working budget

Budget an HMI screen color palette at roughly 20 distinguishable entries. Engineers who have stepped through a full color palette on an installed panel count about 20 entries that read as clearly different colors on the glass, and the observation agrees with how vision works: the eye separates steps in lightness far more easily than small steps in hue, and most palette entries sit close together in hue with little lightness between them.

Three things shrink the budget on the floor: a dimmed backlight, glare on the bezel, and viewing distance. One thing widens it: lightness contrast. Two entries that differ in both hue and lightness stay separate where two near-neighbor hues merge, so spread your entries across lightness as well as hue. Two entries separated by at least a 20 % lightness step stay apart even when their hues sit close; a 5 % lightness step merges under glare. Use 20 as the planning figure until the sign-off test on your own panel gives you its number.

What ISA-101 asks for

The process-control screen standard ISA-101 takes the same position from the design side. It asks for screens built mostly in grays, with color held back mainly for abnormal conditions and alarms, and it wants a fixed alarm color set by priority. The usual set covers four priorities, P1 through P4, so one hue means one urgency on every screen in the plant. A light gray background also cuts glare, which is part of why high-performance screens look plain.

The practice fits the budget arithmetic. A gray screen spends almost none of its 20 entries on decoration, and the colors it does spend are the four alarm priorities plus a handful of state colors, which lands the whole screen near a dozen entries with margin to spare.

What this changes on a real screen

• Cap the assigned entries at about a dozen. A dozen sits inside the 20-entry budget with margin for screens other people designed.
• Give every state that demands a reaction both a lightness step and a second cue: shape, text or position. A valve that shows open and closed as two near-neighbor hues at the same lightness will get misread.
• Never encode a safety-relevant difference in hue alone. About 1 man in 12 has reduced red-green color vision, so an interlocked or de-energized state carries text and a shape beside its color.
• On a monochrome panel, use 3 to 5 of its documented gray shades spread across the range: near-white background, mid gray for static objects, near-black text and lines, and the darkest shade for the state nobody may miss.
• Size state text to the distance it is read from: at the usual 6 ft stand-off at a filling line, capitals of at least 0.25 in, following the 30 ft per inch of letter height planning figure, keep every state label readable.
• Keep one meaning per color on every screen in the plant. The cheapest way to hold that line is to build screens from a reusable HMI equipment panel set, so the palette travels with the layout.

For motors and valves, fix the state color set once and drive it from the controller, which is the pattern behind how PLC-driven HMI graphics represent equipment states. And when one object must show several states at once, change the shape or the pattern and let color back it up; showing three alarm states through one indicator works exactly that way.

Example: cutting a 38-entry screen to 11

Scenario: a retrofit screen for a filling line, a 15-inch panel read from about 6 ft. The color panel is documented at 128 palette entries, and the first project pass uses 38 of them: 12 recipe colors, 14 step colors and 12 machine-state colors. Operators complain the step colors look alike, and several step hues differ by less than 10 % in lightness.

Measure that pass against the budget. 38 assigned entries stand against roughly 20 distinguishable ones, so 38 - 20 = 18 entries exceed what operators can reliably separate on the panel, and the step group alone, at 14 entries, spends most of the budget on hues that look alike.

Redesign to semantic colors:

• 3 grays: light gray background, mid gray for vessels and pipework, dark gray for text and outlines
• 4 alarm colors, one per priority, P1 through P4
• 2 abnormal state colors: unexpected stop and blocked
• 1 accent color for the object the operator must watch now
• 1 disabled shade for equipment out of service

The count lands at 3 + 4 + 2 + 1 + 1 = 11 entries, which leaves 20 - 11 = 9 entries of margin. Recipes separate by the text on the vessel and steps by the step number in the header, so neither group needs a hue of its own.

The result: the screen fits the budget with 9 entries spare, every operator-critical state carries a second cue, and the design transfers to a later monochrome station, where 4 gray shades spread across the documented 16, roughly every fourth shade, carry background, objects, text and the darkest alarm color.

Sign off on the installed panel

An editor display differs from the panel in backlight, gamma and surface, so colors that separate on a desk can merge on the machine. Judge the palette where it will run:

1. Load the revised screens onto the installed panel.
2. Step through every state of every operator-critical object, including adjacent palette entries, because those merge first. On a screen with 40 states this pass takes about 10 minutes.
3. Have someone who did not build the screens name each state out loud, without using the color names from the project.
4. Run the pass again at the lowest backlight setting the floor will use, where dimming and glare compress the differences.

Two entries that read the same get a lightness step or a second cue, and the release waits until every state is named correctly.

One fault imitates a palette limit and is a configuration problem instead: after a download, a runtime client renders palette entries black. That fault sits in the client's palette handling and has a documented fix of its own, described in palette entries that render black on a runtime client. Clear it first, then judge colors on the glass.

Recommendation

Recommendation: Size every operator-facing palette to the eye, not the spec sheet. Budget roughly 20 distinguishable entries, assign no more than about a dozen, build the screen in grays after the ISA-101 practice, and spend color on the four alarm priorities plus the two or three states that demand action. Give every safety-relevant state a text or shape cue beside its color, and on a monochrome panel use 3 to 5 gray shades spread across the documented range. Release the screen only after someone who did not build it has named every critical state correctly on the installed panel, at the brightness it runs at.

Questions