There is a familiar pressure on every project to get to colour quickly. Colour reads as progress. Stakeholders can see it, respond to it, feel that something is happening. Grey boxes look like a designer stalling.
But structure is the only decision in a screen that is genuinely expensive to reverse. Type can be retuned in an afternoon. Colour is a token change. Which regions exist, what they contain, and in what order someone reads them is the thing that engineering builds against and that people form habits around.
Deciding structure while it is still cheap is most of the job. Everything after it is refinement.
Grey removes the anaesthetic
A good arrangement in grey is a good arrangement. A weak one in grey is visibly weak, because there is nothing available to rescue it: no accent colour to redirect the eye, no illustration to fill the awkward corner, no shadow to fake a hierarchy the layout has not earned.
That is precisely why grey is worth defending. It is a test with the anaesthetic removed. If the hierarchy only works once the primary button turns blue, the hierarchy does not work.
Three, not one
The other habit worth keeping is producing more than one. A single wireframe invites feedback on details. Three genuine alternatives invite a decision about structure, which is the conversation you actually want.
On a recent programme we tested three: list-first, split browse, and record-first. List-first was the fastest to build and closest to the legacy tool, which is exactly why people kept leaving it to find context elsewhere. Split browse suited one role and punished four. Record-first won because it matched how every role described arriving at the system, with a name already in mind.
None of that would have surfaced from a polished comp. It surfaced because the alternatives were cheap enough to make, and plain enough that nobody was distracted by whether they liked the blue.
So: grey is not the stage before the design. It is the stage where the design is decided. What follows is resolution, and resolution is the easy part.