Short answer
Plastics are coloured by mixing pigments or dyes into the molten polymer, usually as a concentrated masterbatch; titanium dioxide makes whites and opaque colours, carbon black makes blacks and greys. The finished colour depends on the base resin's own slight tint, the pigment dispersion, wall thickness and the mould's surface texture. White and pale plastics yellow because ultraviolet light and heat oxidise the polymer and its additives into coloured compounds; how fast depends on the polymer, its stabilisers and the exposure.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Photo-oxidative yellowing | Whites and pale greys turn cream or yellow; PVC can go orange | UV forming coloured degradation products in the polymer | UV stabilisers and absorbers; darker or carbon-black grades outdoors |
| Chalking | Pale, powdery surface | Surface polymer erodes and releases pigment | Weathering-grade compounds |
Most coloured plastic parts start as natural (uncoloured) resin plus a few per cent of masterbatch — pellets of the same or a compatible polymer carrying a high concentration of colourant. Pigments are insoluble particles: titanium dioxide scatters strongly and gives whites, pastels and opacity; carbon black absorbs across the spectrum and also screens UV; inorganic pigments give earthy and durable colours; organic pigments give brighter hues. Dyes dissolve in some transparent polymers such as polycarbonate and acrylic to give clear tints. Because the resin itself is rarely perfectly colourless — many are faintly yellow — whites often include a trace of blue pigment or an optical brightener to neutralise it, which is one reason some white plastics look bluish in daylight.
The same compound can look different on different parts. Thin walls let light through, so a pale part looks greyer or shows internal ribs where it is thin, while thick sections look deeper. Mould texture changes the balance of surface reflection: a high-polish tool gives a darker, more saturated look; a spark-eroded or etched texture scatters more white light and looks paler. Processing temperature and time can degrade some pigments and polymers, shifting colour between machines. Pigment dispersion matters too: poorly dispersed pigment gives streaks and weaker colour. For a multi-part product, colour must be approved per material, texture and wall thickness, and plastics are often metameric against painted or textile parts made to the 'same' reference.
Ultraviolet light absorbed by the polymer or by impurities starts a chain of oxidation reactions that break chains and form new chemical groups, some of which absorb blue and look yellow. PVC is the classic case: on weathering its surface goes yellow to orange as conjugated sequences form along degraded chains, and as the surface breaks down it releases its titanium dioxide, which shows as chalking. ABS and polycarbonate yellow indoors too, especially near windows and under fluorescent lamps, and heat from electronics accelerates it — hence the familiar yellowed computer cases. Stabilisers (UV absorbers and hindered-amine light stabilisers) slow the process; carbon black is a very effective UV screen, one reason black plastics weather well.
Colour stability is a specification choice. Outdoors, weathering-grade compounds with UV stabilisers, pigments chosen for lightfastness and, where possible, darker colours last longest. Indoors, pale and white parts near windows or heat sources benefit from stabilised grades and from avoiding flame-retardant or impact-modifier systems known to yellow; note that some antioxidants add a faint warm tint of their own, so the stabiliser package belongs in the colour approval. Surface coatings and in-mould films can protect colour but add cost. The recycled-plastic pages cover the additional grey and yellow that reprocessing brings. A practical test is accelerated weathering of real parts alongside the reference, measured before and after, because published resin data rarely predicts the colour change of a specific compound.
Why: Photo-oxidation from light and heat, often in flame-retarded ABS or polycarbonate.
Fix: Stabilised or UV-resistant grades for new parts; yellowing is chemical and not reliably reversible.
Why: Surface texture changes how much white surface reflection is scattered.
Fix: Approve colour separately for each texture, or use one texture on adjacent parts.
Why: Surface polymer eroded by UV, exposing and releasing pigment (chalking).
Fix: Weathering-grade compounds; darker or carbon-black pigmented colours last longer.
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.
FactStrong evidence
UV-induced autocatalytic photo-oxidation of plastics causes surface discolouration and cracking along with loss of mechanical integrity over time.
Source: Effects of UV radiation on natural and synthetic materials (Photochem. Photobiol. Sci.)
FactStrong evidence
Weathered PVC surfaces turn yellow to orange as unsaturated sequences form along the polymer, and surface degradation releases the rutile titanium dioxide opacifier, producing chalking.
Source: Effects of UV radiation on natural and synthetic materials (Photochem. Photobiol. Sci.)
FactModerate evidence
Stabilisers can themselves tint a plastic: in a multiple-extrusion study of polyethylene, unstabilised and phosphite-stabilised material stayed near white while a hindered-phenol antioxidant gave a slight yellow-red tint.
Caveat: One laboratory study on repeated extrusion of one polymer.
FactStrong evidence
Light damage in conservation terms includes cracking and chalking of most plastics, resins and varnishes, driven largely by ultraviolet exposure.
Source: Agent of deterioration: light, ultraviolet and infrared
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.