Short answer
Recycled plastic is recoloured the same way as virgin — usually by blending in a concentrated colour masterbatch during moulding or extrusion — but the recipe has to work on a base that is already tinted, grey or hazy and that changes from lot to lot. How hard that is depends on how far the target colour is from the base, whether the part is opaque or transparent, and whether colour must be held exactly or allowed to move with the base.
Most plastics are coloured by adding a masterbatch — pigments or dyes already dispersed at high concentration in a carrier polymer — to natural resin at a set letdown ratio as the material is moulded or extruded. It keeps pigment handling out of the moulding shop, gives good dispersion and lets one resin be made in many colours. With virgin resin, the base is close to colourless and constant, so a fixed recipe at a fixed ratio gives the same colour every time. With recycled resin, the masterbatch is fighting a base whose own colour, haze and flow vary, and that is where the difficulty starts.
Colour mixing on a coloured base is subtractive: the pigment adds its absorption to whatever the base already absorbs. A base with a yellow cast pushes blues towards green and makes reds look browner; a grey base lowers lightness and chroma of everything on it. The recipe therefore has to include correction — extra white to lift lightness, a complementary toner to cancel a cast, more of the chromatic pigment to recover chroma. Recycler guidance notes that transparent light-blue PET is accepted in the clear stream partly because it acts as a bluing agent against yellowness; the same logic is used deliberately when a small amount of blue or violet toner is added to recycled clear material.
For opaque parts, the job is to hide the base completely. The darker and more variable the base, the more pigment is needed before lot-to-lot changes stop showing through, and pale target colours need the most, because they rely on white pigment to create lightness the base does not have. That raises cost and can affect mechanical properties. For dark targets the base helps rather than hinders: mixed-colour polyolefin recyclate already sits near dark grey, so a charcoal or black needs relatively little extra pigment — but in packaging a carbon-black recipe brings the sorting problem that NIR-detectable black pigments were developed to avoid.
There are two ways to run recycled colour. With a fixed recipe, the same masterbatch at the same ratio is added to every lot, and the final colour moves with the base; this is cheap and suits products where some variation is acceptable. With a fixed colour, each incoming lot is measured and the recipe or ratio is adjusted to hit the target, which needs a colour lab, measured lots and more expensive, sometimes custom, masterbatch. Many recycled-content products use a hybrid: blending large base lots to reduce variation, then a fixed recipe with a tolerance wide enough to accept what remains. Whichever is chosen should be written into the colour specification so the supplier knows what is expected.
Why: Yellowish base colour shifting the blue towards green.
Fix: Add a violet or red correction to the recipe, or choose a feedstock with lower b*.
Why: Fixed recipe on a base whose lightness varies.
Fix: Measure incoming lots and adjust white loading, blend larger lots, or move to a darker target.
Why: Carbon-black masterbatch makes the pack invisible to NIR.
Fix: Switch to an NIR-detectable black masterbatch and have the pack sortation-tested.
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.
ConventionModerate evidence· North America
Recycler guidance notes that transparent blue PET is most often included with the clear stream to act as a bluing agent.
Source: APR Design Guide for Plastics Recyclability: PET Rigid
FactModerate evidence· United Kingdom
Reducing the pigment level in cap masterbatch was identified as the most significant quick win for reducing the green hue of recycled HDPE from milk bottles.
FactModerate evidence
NIR-detectable black masterbatches allow dark recycled-content packaging to be coloured black without the carbon black that prevents NIR sorting.
Source: New Near-Infrared Detectable Black Masterbatch; Black plastic packaging hub
Colourwise interpretationModerate evidence
On recycled plastic, a fixed masterbatch recipe produces a colour that moves with the base material, while holding a fixed colour requires measuring each lot and adjusting the recipe.
Based on: Follows from subtractive mixing on a variable base, as documented for recycled PET and HDPE colour variation in the cited sources.
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.