Short answer
Depending on the metal: copper, brass and bronze tarnish and form patina; lacquered fittings discolour where their lacquer has failed; stainless steel and aluminium usually pick up surface deposits rather than changing themselves; dyed anodised aluminium can fade in sun; and weathering steel is designed to rust to brown. First find out whether the colour is on the metal or in it — a wash tells you.
1. Normal tarnish and patina on copper alloys
Test: Look at protected areas (under a fixing, inside a recess). If the protected area keeps the bright metal colour and exposed areas are brown, dark or green, it is patina forming from the surface.
Copper and its alloys react with air, moisture and pollutants: first a brown-to-dark oxide, then, outdoors over years, green copper minerals.
2. A protective lacquer has failed in patches
Test: Look for sharp-edged dark or spotty areas surrounded by still-bright metal. Patchy change on a lacquered brass fitting usually means the lacquer has worn or cracked.
Factory lacquer holds the colour until it is scratched or broken down; tarnish then grows only where it has gone.
3. Surface deposits or staining rather than change in the metal
Test: Wipe a small area with a damp cloth and mild detergent. If the discolouration comes off, it was dirt, salt or run-off rather than corrosion.
Stainless steel near the sea or roads picks up brown staining from deposits; limescale and cleaning residues can dull any metal.
4. Fading of dyed anodised aluminium
Test: Compare the sunlit face with a shaded or hidden face of the same part. Paler colour only on the sunny face points to light fading of the dye.
Dyed anodising takes organic dyes into its pores; some colours, especially reds and blues, fade in sun while blacks and golds hold better.
5. Weathering steel still developing its patina
Test: Check how old the steel is and whether it gets wet and dries regularly. Patchy orange on new work, or dark and flaky rust in a permanently damp spot, tells you which.
Weathering steel is designed to rust to an even dark brown over a few years, but only where it is alternately wetted and dried.
Causes are ordered by how often Colourwise expects each to be the explanation — an editorial judgement, so test them all.
Wash a small area with warm water and mild detergent, rinse and dry it. If the colour comes away, it was a deposit — road grime, sea salt, limescale, run-off from something above, cleaning residue — and the fix is cleaning and, outdoors, a regular washing routine. If it does not, the surface of the metal has reacted. Next, compare exposed and protected areas: under a fixing, inside a recess, behind a handle. Change that stops sharply at a protected edge is surface chemistry driven by air, moisture and pollution. Change that is patchy and sharp-edged on a fitting that was bright when new usually means a clear lacquer has failed in places, letting tarnish grow only where it has gone.
Copper alloys change colour in stages. Indoors they darken from bright metal to brown through a thin oxide layer; outdoors over years they develop the familiar green patina, which on copper roofs is mainly a copper sulphate mineral over a thin reddish oxide layer, with other minerals where sulphur or chlorides are high. The patina is not uniform where the metal has inclusions or where water runs off unevenly, which is why streaks and blotches are normal on newer roofs and cladding. Bronze outdoors can also stain the stone beneath it as pollutant gases turn its surface into soluble compounds. Whether any of this is a problem is a choice: indoors, polishing and a clear finish keep brass bright; outdoors, patina is protective and often intended.
Stainless steel owes its appearance to a very thin, self-repairing chromium-oxide film; when it looks brown or dull it is usually carrying deposits, which wash off, although in coastal or polluted air those deposits can start small pits that are harder to remove. Anodised aluminium takes its colour either from dyes held in the porous oxide — some of which, notably reds and blues, fade in sun while blacks and golds hold better — or from integral and electrolytic processes whose shade varies with the alloy, so replacement parts may never match exactly. Weathering steel is meant to rust: it usually reaches an even dark brown within a few years, but only where it wets and dries in turn, and it stains surfaces below it most in the early months.
Heritage bronze, statuary and designed patinas are a specialist field: a polish can remove, in minutes, a surface that took decades or a skilled finisher to create.
Decide what you want before touching the surface. For household brass and copper, a polish made for that metal, used as its instructions say, restores brightness, and a clear protective finish slows the return of tarnish. For lacquered fittings with patchy failure, polishing only the bare areas leaves a mismatch; replacing or professionally refinishing the fitting is usually neater. For outdoor copper and weathering steel, work with the patina: wash off run-off stains from surrounding surfaces and let the metal even out. For faded anodising, the colour is gone; replacements in a more lightfast colour are the practical route. Anything historic, sculptural or architectural should go to a metals conservator or the original fabricator.
Why: Polish removed tarnish in bare areas but not under surviving lacquer
Fix: Have the fitting stripped and refinished professionally, or replace it
Why: Early run-off while the patina forms
Fix: Plan drainage or sacrificial surfaces; staining lessens as the patina matures
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Historic copper roof patinas consist of a thin inner cuprite layer and a thicker outer layer mostly of brochantite, with antlerite in more sulphur-rich air and atacamite where chlorides dominate; inclusions in the copper make the patina less uniform.
FactModerate evidence
Organic dyes in anodised aluminium can give many hues, but some colours such as reds and blues are particularly prone to fading, while blacks and inorganic golds are more lightfast.
Source: Anodizing
FactModerate evidence
Weathering steel usually attains its uniform dark brown within two to five years, needs alternate wetting and drying to form its protective patina, and is most likely to stain adjacent surfaces in its early months.
Source: Weathering steel
FactStrong evidence
Pollutant gases turn exposed bronze into soluble minerals that can stain the masonry below it.
Source: Preservation Brief 48: Preserving Grave Markers in Historic Cemeteries
Colourwise interpretationLimited evidence
A patchy, sharp-edged change on a fitting that was uniformly bright when new points to failed lacquer rather than to the metal itself.
Based on: Colourwise reading of how clear coatings protect metals: tarnish grows only where the coating is breached, producing edges that follow scratches and wear rather than exposure.
Caveat: Uneven cleaning residues can produce similar patterns; the wash test separates them.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.