What happens to colors on a monochrome HMI grayscale conversion

What the conversion does to each color

A monochrome panel renders each configured color as the gray of matching lightness. Lightness is set by relative luminance, and the WCAG contrast formula weights the three channels 0.2126 for red, 0.7152 for green and 0.0722 for blue. That weighting explains the ranking you will see after conversion: saturated blue lands close to black, pure red lands as a mid gray, yellow and white stay light, and the mid-tone colors crowd into the middle of the 0 to 255 gray scale. Collisions happen in that crowd.

The formula gives you the estimate. The simulator shows the shades the panel actually renders, and that view is where you judge the result. What a color panel's palette can and cannot display is its own subject: see how many colors an HMI screen palette really shows.

The symptoms to hunt for

Walk the converted screens and look for five failures:

• Two state colors read as one shade. The distinction rode on hue; measure the pair's gray contrast as shown in the example below.
• Text over a filled box turns illegible. The text color and the fill converted to nearby grays; recheck every text-over-fill pair.
• A selected button looks unselected. The selection state used hue; it needs a border, an inverted fill or a symbol change.
• Trend lines or legend entries cannot be matched up. The series were distinguished only by color; they need labels, markers or fill patterns.
• A disabled control reads as enabled. Enabled and disabled converted to similar shades. Process HMI style guides call for the opposite: a grayscale disabled control whose foreground is lighter than the enabled control's, so the disabled state never passes as live.

Run the monochrome simulator before you edit

1. Copy the project first. Work on the copy and keep the color original; it is the rollback if the monochrome review stalls.
2. Open the monochrome preview. In the desktop software choose Simulate > Select Monochrome model, pick the model that matches the target unit (the monochrome models are EA7-S6M and S6M-R), then choose Start.
3. Walk the operator screens. Alarm screens, mode indicators, permissives, buttons and trends, plus the pop-up and banner screens, which inherit the same conversion.
4. Judge in the simulator view. The color editor still shows the original hues; the simulator shows what the operator gets.
5. Measure the critical pairs. Take each pair that separates a normal from an abnormal state and compute its gray contrast; the threshold to meet is 3:1, the WCAG minimum for graphical objects.

Example: red and green stop-run colors after conversion

Inputs. Red RGB 255, 0, 0 and green RGB 0, 128, 0, the classic stop and run pair on a color project (illustrative colors for this check). Channel weights 0.2126, 0.7152, 0.0722 and the threshold 3:1, from the WCAG formula. The sRGB linearization step: for a channel value c on a 0 to 255 scale, c_lin = ((c/255 + 0.055)/1.055)^2.4 when c/255 > 0.04045, otherwise c_lin = (c/255)/12.92.

Rule. Linearize each channel, multiply by its weight, and add the three products. The sum is the color's luminance L on a 0 to 1 scale, and the gray it converts to is the 8-bit gray with the same L. Two shades separate cleanly when (L_light + 0.05) / (L_dark + 0.05) reaches 3:1.

Arithmetic. Red: c/255 = 1.0, so c_lin = 1.0. L_red = 0.2126 × 1.0 = 0.2126. Green: c/255 = 0.502, so c_lin = ((0.502 + 0.055)/1.055)^2.4 = (0.528)^2.4 ≈ 0.216. L_green = 0.7152 × 0.216 = 0.154. Gray for red: 255 × (1.055 × 0.2126^(1/2.4) − 0.055) = 255 × (1.055 × 0.525 − 0.055) ≈ 127. Gray for green: 255 × (1.055 × 0.154^(1/2.4) − 0.055) = 255 × (1.055 × 0.459 − 0.055) ≈ 110. Contrast: (0.2126 + 0.05)/(0.154 + 0.05) = 0.2626/0.204 ≈ 1.3.

Result. Pure red converts to gray level 127, the green to gray 110. On a 0 to 255 scale the two shades sit 17 levels apart, a contrast of about 1.3:1.

Decision. 1.3:1 sits well below the 3:1 threshold for graphical objects, so the stop and run pair fails the check: an operator who memorized the colors cannot tell them apart once they convert. Verify the estimated shades in the simulator, then treat any pair that measures under 3:1 as a defect to fix before the project loads.

Fix each collapsed pair with a non-color cue

A hue distinction is the weakest cue on any panel to begin with: red-green color vision deficiency affects roughly 1 in 12 men. The cues that survive a monochrome conversion are words and geometry:

• State text. Put the word in the object: STOP and RUN inside the tile itself. Multi-state text or lookup text is the mechanism for one object that changes its word with its state.
• Drive the words from the PLC. Show two text states on an HMI from one PLC bit ties the wording to the same bit that drives the color, so text and shade can never disagree.
• Fill patterns. Solid fill for one state, hatched for the other. Pattern is geometry, so it converts unchanged.
• Symbols and borders. A crossed symbol on fault states, a heavy border or inverted fill on the selected button.
• Tag-driven color changes. Where a display should change color with its alarm tag, wire that change to the tag as described in change a numeric display color from an alarm tag, then recheck the pair in the simulator.

Size any added text for the distance it is read from: the legibility planning figure is 30 ft of viewing distance per inch of capital letter height, so a tile read from 15 ft needs capitals about half an inch tall.

One rule overrides the rest: if a critical alarm pair still cannot be told apart after these changes, hold the panel out of service until it can. An operator acting on an unreadable state is the failure this whole check exists to prevent.

Verify, then load

1. Re-run the monochrome simulator after each batch of edits. A fix on one screen leaves the same color scheme in place elsewhere; walk every screen again, not just the edited one.
2. Compare the edited copy against the original color project object by object. The edits should touch text, borders, fills and symbols only: no tags, no alarms, no logic.
3. Load the project to the panel and repeat the critical screen checks on the physical unit, from the operator's position and under the shop lighting. The panel is the final readability check.
4. Keep the color original as the rollback copy until the monochrome unit has passed these checks running in the cell.

For the wider set of screen checks on the floor, see factory and shop-floor screens.

Recommendation: keep the color project as the master copy, run the monochrome simulator before touching any object, and rebuild only the objects whose state pairs measure under 3:1 contrast in gray. On a typical project that is a handful of alarm tiles, buttons and trend legends, not a redesign.

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