Short answer
They are coloured by different surfaces. Stainless steel is bare metal seen through a transparent chromium-oxide film only nanometres thick, so its colour is mostly what it reflects and its finish decides how. Weathering steel is deliberately allowed to rust, forming a dense iron-oxide layer that turns from orange to dark brown over a few years. Galvanised steel is coated in zinc, which dulls from bright spangle to matt grey as zinc corrosion products form. The finish, the climate and the direction of any brushing all change what you see.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Weathering-steel patina | Orange to uniform dark brown over roughly 2–5 years | Alternate wetting and drying | Detail run-off away from paving and pale cladding |
| Surface staining of stainless steel | Brown 'tea staining' in coastal and polluted sites | Chlorides and deposits on rougher finishes | Smoother finish, higher-alloy grade, rain-washed detailing, cleaning |
Colours that work with steel (stainless, weathering, galvanised) in a room
Stainless steels contain at least about 10.5% chromium, which reacts with oxygen to form a very thin, transparent, self-repairing oxide film. Because the film is far thinner than a wavelength of light, it does not colour the metal; what you see is the steel's slightly warm, grey metallic reflection. Metals reflect light at the surface without letting it penetrate, so a polished stainless panel is effectively a mirror of the room, and its 'colour' shifts with what is around it. Finishes change that: a mill (2B) surface is a soft grey sheen; a No. 4 brushed finish scores parallel lines that stretch reflections along one axis; a No. 8 mirror finish shows sharp images. Brushing direction therefore has to be consistent across adjacent panels.
When stainless steel is heated — near a weld, or deliberately in a furnace or chemical bath — the oxide film thickens. Once it approaches the thickness of a fraction of a wavelength, light reflecting from its top and bottom surfaces interferes, and the surface shows straw, bronze, purple and blue in sequence as the film grows. Weld heat tint is a sign the protective film has been disturbed and is normally removed by pickling. Controlled chemical colouring uses the same interference principle to make durable gold, bronze, blue and black stainless for architecture; because the colour comes from film thickness, it shifts with viewing angle and is very hard to match between batches.
Weathering steels contain small additions of copper, chromium and nickel that make the rust layer dense and adherent rather than flaking. The patina forms only under alternate wetting and drying: at first it is a bright, uneven orange-brown, and within about two to five years it usually becomes a uniform dark brown, darker and faster in sulphur-polluted air than in clean rural sites. In continuously damp, sheltered or chloride-laden locations — within a couple of kilometres of the sea, for instance — the protective patina may not form properly and the surface can stay coarse and flaky. Run-off in the early months carries rust onto anything below, staining paving, glazing and pale stone.
Hot-dip galvanising coats steel with zinc, often showing large crystalline 'spangle' when new. Zinc is chemically active in air: it forms oxide, hydroxide and then basic zinc carbonate, and the surface dulls within months from bright metallic to a matt, even, bluish or mid grey that most people associate with old galvanised railings. Historic cast-zinc monuments show the same bluish-grey. Painting galvanised steel requires the right primer, and fresh galvanising and weathered galvanising look so different that repairs with zinc-rich paint are visible at first and blend only as the surrounding zinc ages. White, powdery 'wet storage stain' forms if new galvanised work is stacked damp.
Why: Brushing direction differs between panels.
Fix: Specify and mark grain direction; install all panels the same way.
Why: Early run-off before the patina stabilises.
Fix: Pre-weather the steel, detail drainage away from porous paving, or accept temporary staining.
Why: Chlorides and deposits on a rougher finish or lower-alloy grade.
Fix: Smoother finish, a more resistant grade and regular washing.
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.
FactModerate evidence
Stainless steel contains at least about 10.5% chromium, which forms a microscopically thin, self-repairing chromium-oxide film; finish designations include No. 2B, No. 4 (brushed) and No. 8 (mirror).
Source: Stainless steel
FactModerate evidence
Weathering steel usually attains its uniform dark brown colour within 2–5 years; the protective patina requires alternate wetting and drying and does not form reliably in continuously wet, sheltered or high-chloride sites.
Caveat: Industry design guidance; local climate and detailing change the timescale.
Source: Weathering steel
FactModerate evidence
Run-off staining of adjacent surfaces from weathering steel is most likely in the early months, when the corrosion rate is highest.
Source: Weathering steel
FactModerate evidence
Cast zinc is generally bluish-grey in colour.
Source: Preservation Brief 48: Preserving Grave Markers in Historic Cemeteries
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.