Short answer
Because matching a colour across different materials, finishes and batches is harder than matching it once. The usual culprits are metamerism (parts that match under the factory light and not under yours), a difference in gloss or texture that reads as a colour difference, tolerances that each part meets but that add up when they touch, and parts that age at different rates. Changing the light and the viewing angle separates them.
1. Different materials matched under one light only (metamerism)
Test: Carry the two parts together from a window to a warm lamp and to a cool-white LED. If the difference appears, disappears or reverses as the light changes, they are a metameric pair.
Different pigments or dyes in different materials can reach the same appearance under one light and part company under another.
2. Different gloss or texture on the same colour
Test: Look at the parts face-on, then at a low angle towards a window. If they match face-on and differ at an angle, the finish is the difference.
A glossier part looks deeper away from the highlight and paler at it; a textured one scatters differently. The colourant can be identical.
3. Different batches or suppliers within tolerance but not side by side
Test: Check part numbers, labels or production dates. Compare with other examples from each batch.
Each part is made to a tolerance; two parts at opposite ends of it can be visibly different when they touch.
4. Different ageing: one part faded or yellowed faster
Test: Compare hidden areas of both parts. If they matched where hidden but not where exposed, one material has aged faster.
Plastics, dyes and coatings age at different rates, so a set that matched when new drifts apart in use.
Causes are ordered by how often Colourwise expects each to be the explanation — an editorial judgement, so test them all.
If parts that were approved together look different in your home or shop, first carry them together between lights: a window, a warm lamp and a cool-white LED. A pair whose difference appears, disappears or even reverses as the light changes is metameric — the two materials reach the same colour under one light by reflecting different mixtures of wavelengths. This is common whenever different colourants are used in different materials, as with a painted metal panel and a moulded plastic trim, or cotton and polyester in one garment. No amount of adjusting under the original light will fix it; the match has to be made spectrally closer, or approved under the lights the product will actually be seen in.
If the parts match when you look straight at them but differ at an angle, the colour may be identical and the finish different. A glossier part looks deeper away from its highlight and paler within it; a textured or matt part scatters light in all directions and sits somewhere between. Surface texture and haze are separate properties from gloss, and a standard gloss meter does not detect orange peel or haze, so parts can pass a gloss specification and still look different. Measuring instruments also treat surface reflection in different ways — with the specular component included or excluded, or with a 45-degree geometry — so numbers from different instruments on glossy or textured parts cannot be compared with one tolerance.
The finish simulator shows the same pigment in matt and gloss side by side, which is often enough to settle whether a complaint is about colour or about sheen.
Each part is made to a tolerance around the target, and two parts at opposite edges of the same tolerance can be further apart from each other than either is from the target. Where they touch, with a straight seam between them, the eye is at its most sensitive: differences of one or two CIEDE2000 units are visible side by side on large flat samples. Textile industries often use the CMC formula with a 2:1 weighting for acceptability decisions, while print and plastics tend to use CIEDE2000 — and tolerances tighter than the agreement between the supplier's and customer's instruments cannot be enforced. Check part numbers and batch codes: parts from one batch are more likely to match than parts from two.
A colour name or a single reference number is not a specification. A workable one names the target (ideally a physical standard plus measured values), the illuminant and observer for measurement, the colour-difference formula and the tolerance, the gloss level and geometry, and at least one second light under which the parts must still match — a metamerism check. Approve physical samples of every material together, under two or three lights including the one the product will be sold under. For assemblies of different materials, ask suppliers to formulate against the same spectral target rather than the same appearance, which is what reduces metamerism. The same thinking applies, with more variation built in, to parts made with recycled content.
Why: Metameric pair approved under one light
Fix: Approve under the retail light as well, and reformulate one part closer spectrally
Why: Different instruments, geometries or specular settings on a glossy part
Fix: Agree one geometry, specular setting and instrument model, and exchange a physical standard
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
Two specimens with identical tristimulus values for a given illuminant and observer are metameric if their spectral reflectances differ within the visible range.
Source: CIE 080-1989 Special Metamerism Index: Change in Observer; CIE (International Commission on Illumination) publications
StandardStrong evidence
The CIE assesses daylight simulators with a metamerism index computed as the average colour difference of sample pairs that match under reference daylight.
Source: CIE 051.2-1999 A Method for Assessing the Quality of Daylight Simulators for Colorimetry
FactModerate evidence
Surface texture and haze are appearance attributes separate from gloss, and a standard glossmeter's narrow acceptance angle does not detect orange peel or haze.
FactModerate evidence
The CMC(l:c) formula, published in 1984 for the Society of Dyers and Colourists, is commonly used with a 2:1 ratio for acceptability decisions.
Colourwise interpretationModerate evidence
Parts that meet their individual tolerances can still mismatch visibly with each other, because their deviations from the target can lie in opposite directions.
Based on: Colourwise reading of how tolerances work geometrically in colour space: two points each within radius r of a target can be up to 2r apart.
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.