Gray — Framing

Gray is the only non-functional color: legends, boundaries, and annotation that structure the diagram without carrying signal or authority.

What gray means

Gray marks framing — the parts of the blueprint that organize and label rather than function. Layer boundaries, the legend itself, IT/OT separation lines, and explanatory annotations are gray. Nothing gray carries telemetry, commands, or authority.

What lives in gray

• Layer band boundaries between L0 through L7.
• The legend that defines the color grammar and line semantics.
• The IT/OT and zero-trust separation boundary lines.
• Grouping frames and non-functional annotation.

Why framing gets a reserved color

Because the other six colors all carry meaning, framing needs a color that carries none — otherwise a boundary line could be misread as a data path. Reserving gray guarantees that anything structural is visibly inert. An engineer scanning for signal paths can ignore every gray element with confidence.

The most important gray line

The single most consequential gray element is the IT/OT separation boundary. It marks where the plant's operational-technology network — the deterministic control side — is isolated from general information-technology systems. Gray draws that line; the security architecture enforces it with airgaps and unidirectional data diodes so that telemetry can flow out for analysis while nothing can flow in to the control path uninvited.

Framing versus signal, at a glance

The practical test for a gray element is simple: remove it, and no data, command, or authority changes hands — only clarity is lost. That is what distinguishes a layer band or a legend from a real connection. Because framing is visibly inert, a reviewer scanning a blueprint for safety-path violations can skip every gray element and lose nothing.

The security boundary gray represents is detailed under the zero-trust and IT/OT separation model referenced from data lineage and governance and the safety hierarchy.