Short answer
Because recycled feedstock carries its past with it: residual pigments, labels, inks and contamination that change with each delivery, extra heat history that darkens and yellows some polymers, and a recycled-content ratio that may itself vary. A colour target set for virgin material exposes all of it. The fix is usually to move the target towards what the stream does well and to specify lightness and yellowness separately.
1. Residual colour and contamination in the recycled feedstock
Test: Look closely at a part in raking light: specks, streaks or haze point to contamination; an even grey or green-yellow shift points to residual colourants.
Recycled streams carry pigments, labels, inks and fragments from their previous life; the mix changes with each delivery.
2. Heat history: the material has been processed more times
Test: Compare lightness and yellowness across batches with the supplier's recycled-content and reprocessing information.
Each melt-processing cycle can darken and yellow some polymers; a batch with more reprocessed content drifts further.
3. Changing recycled-content ratio
Test: Check the batch certificates: parts with 30% and 100% recycled content will not be the same colour.
Blending ratio directly sets how much of the recycled base colour shows.
4. A colour target that the recycled base cannot reach
Test: Ask whether the specified colour is lighter or cleaner than the recycled base. Bright whites and clear pale colours are the hardest.
A darker or greyer base can be covered by more pigment only up to a point; pale targets expose every variation.
Causes are ordered by how often Colourwise expects each to be the explanation — an editorial judgement, so test them all.
Look at the variation before chasing it. Speckles, streaks and haze visible in raking light point to contamination: fragments of other plastics, labels, inks or unmelted particles. An even shift of the whole part — greyer, greener, yellower — points to residual colourants in the feedstock, which change with where it was collected. A drift that follows production history — darker and yellower batches correlating with more reprocessed content or longer residence in the machine — points to heat history. And a step change between deliveries often means the recycled-content ratio changed. Batch certificates and the supplier's collection-route information usually settle which; the materials topics on recycled colour explain each mechanism in depth.
Recycled streams carry whatever the previous products contained. In recycled PET bottles, haze has been shown to rise linearly with the number of insoluble contaminant particles, and the collection route — bottles collected alone or mixed with other packaging — was identified as the crucial factor for both contamination and colour. In recycled HDPE from milk bottles, a green tint that limited recycled content was traced mainly to pigment in coloured caps and to labels leaching colour during washing. These are supply-chain properties, not moulding faults, which is why the most effective fixes are upstream: tighter sorting, cleaner collection streams, or a supplier whose stream is consistent.
Each time a polymer is melted and reformed, some of it degrades, and for many plastics that shows as darkening and yellowing. In one study of repeated extrusion of PLA, yellowness rose steadily over six cycles and a single extra cycle produced a colour difference large enough for an inexperienced observer to notice; repeated extrusion of bottle-grade PET under controlled conditions likewise darkened it progressively. So a batch with more reprocessed material, or one that sat longer at temperature, will look different even from the same feedstock. Stabiliser choice matters too: some stabilised grades hold their yellowness almost unchanged over several cycles, which is worth asking suppliers about.
A single ΔE number can hide which way the colour moved. For recycled material the direction — darker, or yellower — is usually more useful than the size.
Bright whites and clean pale colours are the hardest targets for recycled content, because every trace of residual colour shows through. Mid to dark colours, slightly greyed tones and deliberately speckled finishes absorb variation well. Where a specification is needed, state the colour-difference formula, illuminant and observer, and set separate limits for lightness and for yellowness rather than one overall ΔE, since recycled bases tend to move along those axes. Agree a range of acceptable batches with physical limit samples at each end. And talk to the compounder early: they can say what colour range their stream reliably achieves, and whether per-batch masterbatch correction is practical.
Why: The target is lighter and cleaner than the recycled base reliably allows
Fix: Move the target darker or greyer, or reduce recycled content for that part
Why: A single ΔE tolerance hides the direction of the shift
Fix: Add a separate yellowness (b* or yellowness index) limit
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 the collection route was identified as the crucial factor for contamination and bottle colour.
FactModerate evidence
In recycled HDPE from milk bottles, a green hue that limited recycled content was traced mainly to pigment in coloured caps and to over-laminated paper labels leaching colour in the wash.
FactModerate evidence
Across six extrusion cycles of PLA, b* rose from 10.75 to around 21 and lightness fell, with a ΔE*ab of 3.6 after a single reprocessing cycle.
Caveat: One grade under laboratory conditions; other polymers and stabiliser packages behave differently.
FactModerate evidence
Repeated extrusion of bottle-grade PET under controlled laboratory conditions produced progressive darkening.
Colourwise interpretationModerate evidence
For recycled content, separate lightness and yellowness limits catch the typical drift better than a single overall ΔE tolerance.
Based on: Colourwise reading of reprocessing studies in which change is dominated by darkening or rising b*, and of the materials pillar's analysis of recycled-colour specifications.
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.