Short answer
Plastics are toleranced on a moulded plaque against the customer's standard, with the formula and limit agreed between supplier and buyer, and the plaque's thickness, surface and moulding conditions fixed as part of the specification. They need fixing because a plastic's colour is partly made in the machine: heat history changes some colourants, mould texture changes the surface, and wall thickness changes how much of a translucent part's colour comes from behind it. A compound can be exactly on formula and still mould off colour.
A coloured plastic part usually passes through a resin maker, a compounder or masterbatch producer, and a converter who moulds or extrudes it, and each needs its own colour check. The inputs vary before anyone processes them: one instrument maker's guide to the industry notes that lot-to-lot variation in colourants is highly possible, and recommends monitoring the strength of incoming pigments and dyes. Dispersion is the second variable. Pigments are insoluble and must be broken down and distributed through the resin, organic pigments being harder to disperse than inorganic ones, and the colour a pigment develops depends on how well that was done. A formula fixes proportions by weight. It does not fix strength or dispersion, which is why batches are corrected from measurement.
Every colourant has a thermal limit, and the same guide describes what happens beyond it: organic pigments fade, and inorganic pigments turn dark and dull. Processing temperature, and how long the melt is held at it, therefore change colour with no change in recipe, and the same compound can differ between two machines or two cycle times. Heat has a second, reversible effect at the point of measurement. A part read warm from the mould has not reached its final colour, and guidance is to track the colour at set times as it cools and compare only at ambient temperature. One benchtop spectrophotometer builds in an infrared sensor for sample temperature for exactly this reason. Resins also yellow with heat and age, which is tracked with a yellowness index.
Plastics are measured as moulded plaques, and two properties of the plaque matter as much as its pigmentation. Surface: a polished face and a grained face of one shot read differently, and for textured parts guidance is to take several readings at different rotations and positions and average them, since one reading spot may not be representative. Instrument geometry interacts with this, a directional 45° instrument responding to surface differences that a sphere instrument partly averages out. Thickness: a translucent grade lets the background contribute, so flat specimens of uniform thickness and a stated backing are needed. Pellets are harder still, being faceted and translucent, and are measured in a backed cup, averaging several fills.
ASTM's practice for calculating colour tolerances names plastic plaques among the specimens it covers and offers CIELAB, CMC, CIE94, DIN99o and CIEDE2000, warning that results in different systems are not directly comparable and requiring purchaser and seller to agree the tolerance and the calculation. It also stresses a point with particular force in plastics: adding the three components into one figure is convenient for a pass or fail, but production needs the direction of the departure in order to correct the batch. In automotive supply the recommended practice for instrumental colour difference includes interior and exterior hard trim in its scope, so that moulded parts are judged on the same footing as the painted and textile parts beside them.
A moulded part is rarely seen alone. It sits beside painted metal, fabric, leather or another plastic, each coloured with a different set of colourants in a different medium, so the match between them is almost always metameric to some degree and has to be checked under more than one light. Gloss and texture differ between the materials as well, which changes how light or dark each one looks beside the others. For that reason a harmony of parts is usually approved as an assembled set in a booth, under daylight and a second source, as well as part by part against numeric limits. A part-level tolerance tells you each piece is near its own standard, not that the pieces agree.
| Comparison | Standard or scheme | Method | Condition | Figure, as the source states it | Caveat |
|---|---|---|---|---|---|
| Moulded plaques and parts against the colour standard | ASTM D2244-25 | CIELAB, CMC, CIE94, DIN99o or CIEDE2000 differences, with the direction of the difference kept | Daylight illumination; opaque specimens such as plaques, painted panels or swatches | None verified; agreed by the parties | The practice warns that differences calculated in different systems are not directly comparable, and that gloss and texture change how a measured difference relates to acceptability. |
| What you see | Where it sits | Mechanism | Test that isolates it | Most likely cause | Next action |
|---|---|---|---|---|---|
| A batch made to the same formula differs from the last one | Process | A formula fixes proportions, not the colour strength of what is weighed. Colourants vary from lot to lot, dispersion varies with mixing, and heat history changes some pigments. | Measure the incoming colourant lot at a fixed concentration against the retained previous lot before blaming the process. | A change of colourant lot or strength, then dispersion, then processing temperature | Check colourant strength on receipt and correct the batch from measurement, not by repeating the recipe. |
| Moulded parts from one compound vary with tool, wall thickness or machine | Process | Mould texture changes surface scattering, a thin translucent wall lets the background through, and excess processing temperature degrades heat-sensitive colourants. | Mould the standard plaque on the same machine at the bottom and top of the temperature window, and measure a textured and a polished face of one shot. | Surface texture first, then thickness in translucent grades, then heat history | Fix the plaque tool, thickness and surface in the specification, and measure only parts cooled to room temperature. |
| A sample measured straight off the line fails, and passes an hour later | Sample | Many strongly coloured materials shift reversibly with temperature, and textiles shift with moisture content until they reach equilibrium with the room. | Measure the same piece at intervals as it cools or conditions and plot the readings against time. | Thermochromism or an unconditioned sample, not a batch fault | Set one measurement temperature and a conditioning time for standards and samples at every site. |
| Two parts in the same colour look lighter and darker beside each other | Sample | A glossier surface sends its surface reflection in one direction, away from the eye, and looks darker and more saturated; a matt one scatters it into the view and looks lighter and greyer. | Measure both with specular included and excluded. If they agree included and differ excluded, the pigmentation matches and the surface does not. | A gloss or texture difference, not a pigmentation difference | Tolerance gloss alongside colour, and fix the surface or tool finish before touching the formula. |
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
An instrument maker's guide to the plastics industry identifies colourant selection, processing temperature and dispersion method as the main factors affecting colour, and notes that lot-to-lot variation in colourants is highly possible.
Source: Color and Appearance Measurement in Plastic Industry (white paper)
FactModerate evidence
Exceeding the thermal limits of colourants causes degradation: organic pigments exhibit colour fading, while inorganic pigments turn dark and dull.
Caveat: A general statement from a manufacturer's white paper; individual pigments have their own heat-stability ratings.
Source: Color and Appearance Measurement in Plastic Industry (white paper)
StandardStrong evidence
ASTM D2244-25 covers colour tolerances and small colour differences between opaque specimens including plastic plaques, in CIELAB, CMC, CIE94, DIN99o or CIEDE2000 units, and notes that production control may require knowing the direction of the departure from the standard as well as the total difference.
ConventionModerate evidence
Guidance for measuring plastics is to average several readings at different rotations and positions on textured mouldings, to use flat specimens of uniform thickness for translucent materials, and to measure pellets in a cup with a backing, averaging several fills.
Source: Enhancing Plastics Manufacturing with Spectrophotometric Color Quality Control
ConventionModerate evidence
Products measured before they have cooled to ambient temperature have not reached their final colour; guidance is to assess the sample at scheduled times during cooling and to avoid comparisons during the cool-down stage.
Source: Thermochromism: The Effects of Temperature on Pigments and Dyes; Datacolor Spectro 1000 series specification sheet
Colourwise interpretationModerate evidence
A part-level tolerance shows that each moulded part is close to its own standard; it does not show that adjacent parts in different materials match each other, which has to be approved on the assembled set under more than one light.
Based on: Follows from the scope of SAE J1545, which spans paint, trim and textiles to give a common basis, and from the way metamerism arises between different colourant sets.
Reviewed 6 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.