You have measured two colour samples with a spectrophotometer. The Delta E is 0.8 well within tolerance. The printed swatch looks identical to the master reference under the light booth. The client approves the proof. Then the swatch arrives at a retail store, sits under fluorescent ceiling panels, and the customer says it does not match the paint tin next to it. The colours that were identical in your controlled environment are visibly different under shop lighting.
This is metamerism one of the most persistent and least understood problems in colour reproduction. If you work in paint colour matching or any discipline where printed colour must represent a physical product, metamerism will affect your work. Understanding it helps you anticipate, manage and communicate the issue before it becomes a client complaint.
Key takeaways
- Metamerism occurs when two colour samples appear identical under one light source but visibly different under another despite measuring as a match under controlled conditions
- The root cause is spectral mismatch: the two samples reflect light at different wavelengths but produce the same perceived colour under a specific illuminant
- Metamerism is inherent in any process where printed ink is used to reproduce a colour originally created with a different colourant system (paint pigments, dyed textiles, plastics)
- The problem cannot be eliminated entirely, but it can be minimised through spectral matching, multi-illuminant assessment and careful ink formulation
- Communicating metamerism risk to clients before production prevents disputes; discovering it after delivery creates them
- The CIE Metamerism Index quantifies the risk, but visual assessment under multiple light sources remains the most practical quality control method
What Causes Metamerism
Every coloured surface has a unique spectral reflectance curve a graph showing how much light it reflects at each wavelength across the visible spectrum (380-730nm). Human colour perception works by combining the responses of three types of cone cells in the eye, each sensitive to a different range of wavelengths. This means that the same perceived colour can be produced by many different spectral curves the eye cannot distinguish between them because it reduces the full spectral information to just three signals.
Recommended Reading
Educational Knowledgebase
When two samples have different spectral curves but produce the same three cone responses under a specific light source, they are a metameric match. They appear identical under that illuminant. But change the light source from D50 daylight to fluorescent, from fluorescent to LED, from LED to tungsten and the balance of wavelengths illuminating the samples changes. The two different spectral curves now produce different cone responses, and the colours that were identical suddenly diverge.
In colour sampling and print reproduction, metamerism is structurally unavoidable because the printed swatch and the original colour are made from fundamentally different colourant systems. A paint colour is created with pigment dispersions in a resin binder. A printed reproduction uses CMYK process inks (or extended gamut inks) on a coated paper substrate. The spectral curves of these two systems will never be identical, even when the perceived colour matches perfectly under a single illuminant.
Where Metamerism Creates Problems
The practical consequences of metamerism surface in situations where the printed reproduction is compared directly to the original product under uncontrolled lighting. The most common problem scenarios include paint fan decks displayed alongside paint tins under retail store lighting, printed colour swatches used by decorators to select colours on site under mixed natural and artificial light, brand colour samples compared to actual products (packaging, textiles, plastics) under varying lighting conditions, and colour specification documents used for quality control across different manufacturing facilities with different lighting environments.
In each case, the same pattern occurs: the printed colour was approved under controlled conditions (typically D50 daylight simulation in a light booth), but the end user encounters it under different conditions where the metameric mismatch becomes visible. The user perceives a colour error “this swatch does not match the paint” when in fact the swatch was accurately matched under the specified assessment conditions.
Measuring Metamerism: The CIE Metamerism Index
The CIE (International Commission on Illumination) defines a Metamerism Index that quantifies the degree of metameric mismatch between two samples. It works by measuring the Delta E between the samples under a reference illuminant (typically D50) and then calculating the Delta E under one or more test illuminants (typically illuminant A for tungsten, F11 for fluorescent, and LED reference spectra).
If the Delta E is low under the reference illuminant (the samples match) but significantly higher under a test illuminant (the samples diverge), the Metamerism Index is high, indicating a strong metameric relationship. A Metamerism Index below 0.5 is considered a good result meaning the match holds reasonably well across different light sources. An index above 2.0 indicates a problematic metameric pair that will show visible differences under common alternative lighting.
The Metamerism Index is useful as a specification tool you can set a tolerance for acceptable metamerism alongside your Delta E tolerance but it does not replace visual assessment. Human colour perception is influenced by surrounding colours, adaptation state, viewing angle and fatigue in ways that instruments cannot fully capture.
Minimising Metamerism in Print Colour Matching
While metamerism cannot be eliminated when reproducing colours across different colourant systems, it can be reduced through several approaches.
Spectral matching rather than colorimetric matching. Standard colour matching targets L*a*b* values (colorimetric match) under a single illuminant. Spectral matching attempts to minimise the difference between the full spectral curves of the sample and reference, not just the perceived colour under one light source. This requires spectral measurement data for the target colour and profiling software capable of spectral optimisation. The result is a printed colour that holds its match across a wider range of illuminants.
Multi-illuminant assessment during proofing. Instead of evaluating proofs solely under D50, assess them under D50, fluorescent (F11 or TL84), LED and tungsten illuminants in a multi-source light booth. If the match breaks down under a specific illuminant that the end user is likely to encounter, the proof can be adjusted before production.
Extended gamut inks. Standard CMYK inks have a limited spectral range. Extended gamut systems (CMYK plus orange, green, violet) provide additional spectral options that allow the profiling software to construct ink combinations with spectral curves closer to the target. This is one of the reasons we use extended gamut printing for colour-critical fan deck production work.
Substrate selection. The substrate contributes to the spectral character of the printed colour. Optical brightening agents (OBAs) in paper stocks fluoresce under UV light, shifting the spectral curve in ways that can worsen metamerism. For colour-critical work, OBA-free substrates produce more stable spectral behaviour across different light sources.
Communicating Metamerism to Clients
Metamerism is a physics problem, not a quality problem. But if the first time a client hears about it is when their customer complaints arrive, it becomes a relationship problem. Proactive communication is essential.
At the specification stage, explain that the printed colour is matched to the physical reference under standard assessment conditions (D50 daylight). Under different lighting particularly fluorescent and LED retail lighting some degree of visual difference may be perceptible. This is inherent in cross-media colour reproduction and is not a defect in the printed output.
Recommended Reading
Educational Knowledgebase
Include a metamerism statement in your colour matching reports. We provide metamerism assessment data with every colour-critical proof, showing the Delta E under D50, fluorescent and LED illuminants. This gives the client the data to make an informed acceptance decision and sets realistic expectations for how the colour will perform in real-world viewing conditions.
For clients who need the closest possible match across multiple lighting environments, we offer spectral optimisation as part of our colour matching service. This reduces (but does not eliminate) metameric mismatch at additional production time and cost.
If you are working on a colour matching project where metamerism is a concern, discuss your colour requirements with our colour team and we will advise on the best approach for your specific application and viewing conditions.
Frequently Asked Questions
Can metamerism be completely eliminated?
Not when reproducing colours across different colourant systems (paint to print, textile to print). The only way to eliminate metamerism entirely is to use the same colourant system for both the reference and the reproduction which is not possible when the reference is paint and the reproduction is ink on paper. Metamerism can be minimised through spectral matching and multi-illuminant assessment, but some residual mismatch will always exist.
Which light source causes the most metamerism problems?
Fluorescent lighting (particularly older tube types with narrow spectral emission bands) tends to cause the most visible metamerism issues. LED lighting varies depending on the LED spectrum early LEDs with poor colour rendering were problematic, while modern high-CRI LEDs cause fewer issues. Tungsten and daylight are generally the most forgiving illuminants for metameric pairs.
Does metamerism affect digital colour proofing?
Yes. A digital proof printed on a different substrate with different inks is a different colourant system from the production output. If the proof matches the production under D50 but the client evaluates it under office fluorescent lighting, they may perceive a colour difference that does not exist under the specified assessment conditions. Always evaluate proofs under the correct illuminant.
How do I specify metamerism tolerance?
Specify a maximum Metamerism Index alongside your Delta E tolerance. For example: “ΔE00 ≤ 2.0 under illuminant D50; Metamerism Index ≤ 1.0 for illuminant F11.” This tells the print provider that the colour must not only match under D50 but must also hold reasonably under fluorescent lighting. The specific illuminants to include depend on where the printed material will be viewed.
Is metamerism the same as colour inconstancy?
No. Metamerism describes the behaviour of a pair of colours (their match changes under different illuminants). Colour inconstancy describes how a single colour appears to change under different illuminants. All colours show some degree of inconstancy a red that looks vivid under tungsten may appear duller under fluorescent but this is a property of the individual colour, not a mismatch between two samples.
Do all paint colours show metamerism when printed?
Virtually all paint-to-print reproductions exhibit some degree of metamerism because the colourant systems are fundamentally different. However, the severity varies by colour. Earth tones, muted colours and colours built from broad-spectrum pigments tend to show less metamerism than highly chromatic colours, fluorescent tones and colours formulated from narrow-band pigments.
