Short answer
Colourants built into the material — oxides dissolved in glass, pigments dispersed in a plastic melt, dyes fixed in fibre — survive mechanical recycling and carry into the next product. Colourants sitting on the surface, such as label inks and paper printing inks, can often be separated. The outcome depends on where the colourant sits, how it is bound, and whether the recycling route melts, pulps or chemically breaks down the material.
Colourants can be divided by location. Bulk colourants are distributed through the material: chromium and iron oxides dissolved in glass, pigment particles dispersed in a moulded plastic, dye molecules bound inside a fibre. Surface colourants sit on top: printing ink on paper, ink on a label, a lacquer on a can. Mechanical recycling — melting, remelting, shredding and re-spinning — has no step that separates bulk colourant from the material it is in, so bulk colour always carries forward. Surface colour can be separated if the recycling process has a step designed for it: flotation for paper ink, float–sink and washing for plastic labels, burning off coatings when metal is remelted.
In container glass the colourant is part of the glass network. Green glass contains a few tenths of a per cent of chromium oxide; amber glass relies on an iron–sulphur complex that forms only under reducing furnace conditions. Remelting does not remove chromium, and a small amount of green cullet in a clear melt is enough to tint it — a WRAP technical study found that 1% green cullet roughly doubles the chromium level of flint glass. The amber chromophore is the partial exception: because it depends on furnace chemistry, an oxidising flint melt can partly bleach amber cullet, which is why amber contamination in flint is less damaging than green.
Pigment particles in plastics are far smaller than the mesh of the melt filters used in recycling, so they stay in the recyclate; dyes dissolved in a polymer stay with it too. Two other residual colour sources are easy to overlook. Degradation products of stabilisers — the quinone-type compounds formed when hindered-phenol antioxidants are consumed — add a yellowish tint that the next processor inherits. And pigmented components recycled with the main part, such as caps on milk bottles, dilute their colour into the whole batch; WRAP traced a green hue in recycled HDPE mainly to that route.
Printing inks on paper mostly sit on and between fibres, so deinking can detach them: pulping in alkaline conditions loosens ink particles and flotation carries them away on air bubbles. Dyes used to colour the paper itself are inside the fibre and need reductive bleaching to strip them. Textiles are the opposite of printed paper: the dye is inside the fibre by design, so mechanical recycling keeps it and recyclers sort by colour instead. Only chemical routes that dissolve or depolymerise the fibre can separate textile dye, and research on glycolysis of coloured PET shows even then the colourant follows the monomer unless an extra purification step removes it.
| Colourant | Where it sits | Mechanical recycling | Route that can remove it |
|---|---|---|---|
| Chromium / iron oxide in glass | Dissolved in glass | Carried forward | None practical; sort cullet |
| Amber Fe–S chromophore | Formed in glass by redox | Carried forward, partly bleachable | Oxidising melt (partial) |
| Pigment in plastic | Dispersed particles | Carried forward | Dissolution or depolymerisation with purification |
| Antioxidant by-products | Dissolved in plastic | Carried forward, may grow | Chemical recycling |
| Label ink on floating film | On separate label | Leaves with label | Float–sink and washing |
| Printing ink on paper | On and between fibres | Removed partly | Deinking (flotation, washing) |
| Dye in paper or textile fibre | Inside fibre | Carried forward | Reductive bleaching; chemical recycling |
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
Adding 1% green cullet roughly doubles the chromium content of flint glass, and green cullet above about 0.5% in flint is typically just noticeable.
Caveat: From a 2004 technical study of UK container glass.
Source: Feasibility Study for the Reduction of Colour within the Glass Furnace (GLA0023)
FactModerate evidence
The amber colour of container glass comes from a complex of ferric iron, reduced sulphur and oxygen and is lost if either iron or sulphur is absent, which is why oxidising melting can partly bleach amber cullet in flint glass.
Source: Feasibility Study for the Reduction of Colour within the Glass Furnace (GLA0023)
FactModerate evidence
In multiple-extrusion experiments on polyethylene, material stabilised with a hindered-phenol antioxidant developed a slight yellowish-reddish tint from quinoid breakdown products, while unstabilised and phosphite-only material stayed close to white.
Caveat: Colour reported graphically; one polymer grade.
FactModerate evidence
In glycolysis of coloured PET waste, colourants discolour the recovered monomer unless an additional extraction step isolates it.
Caveat: One study of packaging waste; commercial processes add dedicated purification.
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.