Short answer
Bare aluminium is a pale, slightly cool metal under a thin natural oxide. Anodising grows a much thicker, porous oxide that can hold dyes, metal deposits or produce interference colour, so the colour sits inside a transparent layer above the metal and keeps a metallic depth. Powder coating and liquid paints instead hide the metal under pigment. Anodised colour depends on the alloy, the oxide thickness, the dye or colouring method and the process, which is why batches and alloys differ; coatings behave like paint.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Dye fading in anodic films | Organic-dyed reds and blues fade outdoors | Light on organic dyes | Electrolytic or inorganic colours outdoors; proper sealing |
| Chalking of coatings | Paler, powdery surface on older powder coats | UV degradation of the binder | Architectural-grade polyester or PVDF systems |
In sulphuric-acid anodising the aluminium part is the anode in an acid bath; the metal converts to aluminium oxide from the surface inwards, forming a layer that is typically a few to about 25 micrometres thick and riddled with fine pores tens of nanometres across. On its own the layer is nearly transparent, so clear-anodised aluminium looks like the metal but slightly greyer and less reflective, and its appearance depends on the metal underneath. Alloying elements and impurities alter both how the oxide grows and its colour: iron, copper and zinc, more common in recycled and some cast alloys, tend to darken or yellow the film, which is why the same process gives different results on different alloys.
Dyeing: organic dyes soak into the pores before sealing, giving almost any hue, including bright reds, blues and purples; some of these fade outdoors, while black and inorganic golds are more lightfast. Electrolytic (two-step) colouring: tin, nickel or cobalt salts are deposited at the bottom of the pores, giving champagne, bronze and black shades that are very light-stable and widely used for windows and cladding. Interference colouring: controlling the deposit so light reflecting from it and from the metal interferes, giving blue, green, yellow or red that shift with angle. Integral colour: special electrolytes and alloys produce bronze to black in the oxide itself, but the shade is so sensitive to alloy metallurgy that it is hard to reproduce consistently.
Most coloured architectural aluminium — window frames, curtain walling, rainwater goods — is not anodised but powder-coated or coated with fluoropolymer paint. These are pigmented films that hide the metal, so their colour behaviour is that of paint: gloss level changes apparent colour, texture finishes read darker, and metallic and mica effect powders introduce angle dependence. Their ageing is binder-led: over years of sun, lower-grade polyester powders lose gloss and chalk, making dark colours look paler and greyer, while architectural polyester and PVDF systems hold colour and gloss much longer. Colour is specified using a paint system's references and gloss level rather than an anodising class.
Anodised colour varies with alloy composition, extrusion versus sheet versus casting, surface pre-treatment (etched satin, bright-dipped, brushed), film thickness, dye bath age, colouring time and sealing. Two parts made from different alloys, or run on different days, can be visibly different even with the same specification. The recycled-metal page covers the extra variation that scrap-based alloys can bring. Industry practice is to agree physical range samples showing the lightest and darkest acceptable parts, keep mating parts to one alloy and one batch, and inspect at more than one angle — especially for electrolytic and interference colours, which change with viewing direction.
| Method | Where the colour sits | Typical colours | Light stability | Angle dependence |
|---|---|---|---|---|
| Organic dye | Dye in the pores | Almost any hue | Varies; some reds and blues fade | Low |
| Electrolytic (two-step) | Metal deposit at the pore base | Champagne, bronze, black | High | Low to moderate |
| Interference | Controlled deposit acting as a thin film | Blue, green, yellow, red | High | High |
| Integral | The oxide itself | Yellow, bronze, grey, black | High | Low; alloy-sensitive |
Why: Different alloys or production batches.
Fix: Specify one alloy and batch for mating parts; agree range samples.
Why: Organic dye with limited lightfastness.
Fix: Use electrolytic or inorganic colours, or a coating, for exterior colour.
Why: UV degradation of a lower-grade polyester binder.
Fix: Specify architectural-grade polyester or PVDF; clean and restore gloss where possible.
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
Sulphuric-acid anodising produces porous oxide coatings typically from about 1.8 to 25 µm thick, with pores roughly 10–150 nm across that can absorb dyes.
Source: Anodizing
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
Integral colour anodising produces yellow, bronze, brown, grey and black shades whose exact colour is sensitive to the metallurgy of the alloy and cannot be reproduced consistently.
Source: Anodizing
FactModerate evidence
Higher iron, copper and zinc contents in aluminium alloys affect anodised appearance, with more iron darkening the anodic coating.
Source: Why Anodizing is the Most Sustainable Surface Treatment for Aluminum
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.