Short answer
Because a recycled batch inherits the colourants, contaminants and heat history of whatever was collected, and that mix changes with every load. How much it varies depends on how finely the feedstock was sorted, whether the colourant can be removed at all, and how many times the material has already been melted, pulped or re-spun.
A virgin polymer, pulp or glass batch is made from controlled raw materials, so its base colour is close to identical every time and any colour is added deliberately afterwards. A recycled batch starts as an average of everything that went into it: bottles dyed blue, green and amber; paper printed with black and process inks; garments in every shade; cans made from different alloys. The colourants in those items were designed to be permanent, so most of them survive recycling. The recycler's first colour decision is therefore not which colour to add but which inputs to keep apart, and the only lever before melting or pulping is sorting.
Almost all colour variation in recycled material traces back to four things. Residual colourant: pigment or dye from the previous life, such as chromium in green glass cullet or pigment in a milk-bottle cap. Contamination: particles, labels, adhesives and food residues that add haze, specks or tints. Degradation: heat and oxygen break polymer chains and create chromophores, so plastics yellow or darken with each melt, and paper fibres carrying lignin yellow. Mixing ratio: the share of each input changes with the season, the collection scheme and what consumers bought, so the average colour drifts even when every step is done correctly. Glass is dominated by residual colourant, polyolefins by mixing, PET by degradation and contamination, and paper by ink and fibre grade.
Colour in recyclate is not just cosmetic. Recycler-industry studies have found that haze in bottles made with recycled PET rises in step with the number of contaminant particles, and that lightness and yellowness move with it; laboratory work on repeated extrusion of PET shows lightness falling in step with intrinsic viscosity, a measure of chain length. In other words, a yellower or darker batch is often telling the processor something about its purity or its degradation. That is why recyclers colour-sort flakes, why reprocessors measure colour on incoming lots, and why a specification that ignores colour may let through material that also performs worse.
Colour sorting at the collection or flake stage is the most effective control: separating clear from coloured PET, natural from coloured HDPE, flint from amber and green glass, and textiles by shade. Better design upstream helps too — lighter-tinted caps, labels that do not bleed, colourants that sorting machines can see. Blending large lots evens out batch-to-batch swings. What cannot be done cheaply is to take the colour back out of a mixed melt: glass colourants are dissolved into the glass itself, and pigment particles in a plastic melt cannot be filtered out. Chemical routes that break a polymer down to monomers can remove colour, at much higher cost and energy.
| Material | Dominant cause | Typical visible effect | Main control |
|---|---|---|---|
| Container glass | Residual colourant (Fe, Cr) | Green tint in clear glass | Colour-separated cullet |
| PET | Heat history and contamination | Yellowing, greying, haze | Flake colour sorting, decontamination |
| HDPE and PP | Mixing of coloured items | Greenish to grey or dark | Natural/coloured separation |
| Paper and board | Ink and fibre grade | Lower brightness, specks | Deinking, grade selection |
| Textiles | Original dye kept in fibre | Shade follows feedstock | Sorting by shade |
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
In PET bottles made with 25–100% recycled content, haze increased linearly with the number of insoluble contaminant particles, and L* and b* also changed linearly with contamination at a rate that differed between rPET sources.
Caveat: One study, three rPET grades, small-scale bottle production.
FactModerate evidence
Repeated extrusion of bottle-grade PET under controlled laboratory conditions produced progressive darkening, with lightness correlating strongly with intrinsic viscosity.
Caveat: Four passes of one clean grade; real recyclate adds contamination effects.
FactStrong evidence
Container glass colourants — iron, chromium and the iron–sulphur amber complex — are dissolved into the glass, which is why colour is controlled by separating cullet by colour before melting.
Source: Feasibility Study for the Reduction of Colour within the Glass Furnace (GLA0023); Glass Color Chemistry
Colourwise interpretationModerate evidence
Across recycled materials, colour variation is best understood as four overlapping causes — residual colourant, contamination, degradation and mixing ratio — with a different one dominating in each material family.
Based on: Colourwise synthesis of the glass, PET, HDPE, paper and textile sources cited across the recycled-materials topics.
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.