Verdigris
Verdigris is the blue-green film that forms on copper and its alloys as they weather. Chemically it is a set of copper carbonates and acetates rather than a single compound, which is why the colour is never quite even and why it varies so much between a sheltered surface and an exposed one. The same patina covers roofs, door furniture, garden urns and the tops of old kettles.
As a decorative colour it was imitated long before it was understood. Painters ground copper compounds into pigments that were notoriously unstable, and decorators learned to reproduce the effect instead with layered washes: a warm ground, a cool green glaze, and a final dry-brushed pale film that catches only the raised parts of a surface.
In a room the colour works best where it is allowed to be uneven. A flat wall painted a single verdigris tone tends to look like nothing in particular; the same green broken over a textured plaster, or used on a piece of furniture where the wear falls naturally on the edges, reads immediately as weathered metal.
It is a mediating colour. Because it contains both blue and yellow it will bridge a room that has drifted into two unrelated palettes, and it is one of the few greens that does not fight with old wood or with terracotta.