The coloured surface layer a material develops as it ages — the green on old copper, the brown on bronze, the soft grey of weathered timber.
Usually a thin film of reaction products formed with air, water and pollutants: copper carbonates, sulfates and chlorides on copper alloys, oxide layers on steel, photo-degraded and leached cell-wall material on wood. Its colour depends on the environment as much as the material.
Patina is not always damage. On copper and weathering steel the layer protects the metal beneath; removing it restarts the process.
Copper roofs brown within months and can take years to decades to turn green.
A material chosen for its new colour will not keep it outdoors; the patina is the colour it will actually be.