A crimson glaze that dims to brown, or a bright wash that quietly loses its intensity, can alter the balance of an entire painting. Pigment lightfastness ratings give painters a practical way to anticipate that risk before colour reaches the palette. They are not a guarantee that every picture will remain unchanged, but they are among the most useful indicators of how a colour may respond to prolonged exposure to light.

For painters making work intended to endure, lightfastness belongs beside hue, opacity, tinting strength and handling. It affects not only what to buy, but where and how a colour is used.

What pigment lightfastness ratings measure

Lightfastness is a pigment’s resistance to fading, darkening or shifting in colour when exposed to light. A highly lightfast pigment holds its essential character under normal display conditions for a long time. A fugitive pigment is more likely to change, sometimes dramatically.

The distinction matters because paints are not simply coloured substances. Each colour is a formulation: pigment dispersed in a binder, with the resulting paint film affected by thickness, ground colour, dilution, glazing and the environment in which it is displayed. The pigment remains the principal source of permanence, but the finished painting is the real object under consideration.

Lightfastness should also be separated from permanence more broadly. Permanence can include resistance to chemical change, humidity, pollutants and interactions with other materials. A pigment may resist light well yet still require sensible use alongside compatible materials and sound painting practice.

How to read pigment lightfastness ratings

Many professional ranges use a scale derived from ASTM testing, where ASTM I denotes excellent lightfastness and ASTM II very good lightfastness. These are generally suitable choices for work expected to be displayed and retained. Lower classifications indicate progressively less resistance and call for more careful judgement.

You may also encounter the Blue Wool scale, particularly in European material information. This measures change against a set of dyed wool standards after controlled exposure. Higher numbers indicate greater resistance to light. Although both systems serve the same purpose, they are not interchangeable labels to be compared without context.

A rating is meaningful only when you know what it describes. Is it assigned to the pigment itself, a particular paint formulation or a diluted tint? Has it been assessed in oil, watercolour or another medium? A lake pigment, for example, may behave differently at full strength than it does when extended into a pale wash or delicate glaze.

That is why the label deserves to be read alongside the Colour Index name, pigment number and any notes on transparency or staining. The tube name is useful, but it does not tell the whole material story.

The role of tint and dilution

A colour can appear perfectly dependable in a thick passage yet become more vulnerable when spread thinly. In watercolour, where transparent washes expose the pigment in a very lean layer, this question becomes especially significant. A pale wash of a less stable yellow may shift sooner than a more concentrated passage of the same colour.

Oil painters face their own variation. A transparent glaze contains less pigment than an opaque, body-colour stroke, while a white admixture changes the optical character of the layer. For this reason, lightfastness testing may consider both masstone and tint. A rating based only on a dense film can conceal a weakness that becomes relevant in a high-key mixture.

This is not an argument against glazing, nor against delicate washes. It is an argument for choosing pigments whose performance suits the way you actually paint.

Why the same hue can have different permanence

Two paints may look almost identical on a colour chart and differ greatly in their long-term behaviour. One may be based on a single, highly stable inorganic pigment; the other may rely on an organic pigment with a more limited resistance to light. Their mixing behaviour, transparency and price may differ too.

Traditional pigment names can be particularly misleading. Terms such as rose, lake, madder, crimson and yellow describe a visual or historical lineage, not necessarily a single pigment. A modern quinacridone may offer the colour family and transparency a painter wants with substantially stronger lightfastness than a historic natural dyestuff.

Equally, not every colour needs to be chosen by lightfastness alone. Some painters knowingly reserve more fugitive materials for studies, sketchbooks, reproduction work or pieces intended to have a temporary life. The essential point is to make that decision deliberately, rather than discovering the compromise after the work has left the studio.

Using ratings to build a dependable palette

A well-chosen palette does not need dozens of colours, but its core colours should be dependable. Begin with the hues that carry the structure of your work: a cool and warm yellow, reds, blues, earths and whites suited to your medium. Check the pigment information for each, especially if you mix large neutral passages or rely on luminous tints.

For landscape painters, the issue often appears in greens and sunlit yellows. A stable yellow mixed with a reliable blue gives greater confidence than a convenient premixed green of uncertain composition. Portrait painters may find it particularly useful to scrutinise transparent reds used in flesh glazes, where a change in one layer can upset the temperature and depth of a whole face.

Single-pigment colours make this process clearer. They allow the painter to understand both the source of the hue and its likely mixing direction. Multi-pigment colours are not inherently inferior, but they require a little more attention: constituent pigments may differ in transparency, tinting strength and lightfastness.

At Michael Harding, pigment information is treated as part of the colour itself, not as an afterthought to the tube label. For a painter, that transparency supports more confident decisions in the shop and at the easel.

Choose by intended use, not by rating alone

An ASTM I colour is not automatically the right choice if its opacity, undertone or handling defeats the purpose of the passage. A painter may need the staining character of a transparent organic pigment, the granulation of a particular watercolour, or the soft covering power of an earth.

The useful question is not simply, “Which colour has the highest rating?” It is, “What is this layer required to do, and which pigment meets that need without introducing avoidable risk?” This is where material knowledge becomes painterly freedom rather than technical caution.

Light exposure is only part of the picture

Even the finest pigments benefit from proper care. Direct sunlight remains demanding, particularly through south-facing windows. Ultraviolet-filtering glazing, sensible hanging positions and stable environmental conditions all help protect a finished work. A painting displayed in harsh, continuous light will age differently from one kept in a moderately lit interior.

Museum framing standards are useful to understand, but they need not make studio practice anxious or precious. The practical aim is straightforward: use sound materials, document what you have used where appropriate, and advise collectors or clients not to hang work in direct sun.

For watercolour and works on paper, protection is especially important. Their transparent passages, pale grounds and often unvarnished surfaces make shifts more visible. Framing under suitable glazing is a material decision, not merely a presentation choice.

Testing colours in your own studio

Published ratings are a valuable starting point, yet painters learn a great deal by making their own exposure tests. Paint a small chart on the same ground you normally use, including full-strength colour, a white tint where appropriate, and diluted washes or glazes. Cover half of each sample with an opaque strip, then place it in a bright but controlled position for several months.

This will not replace laboratory testing, and a sunny windowsill is not a standardised test chamber. It will, however, reveal practical differences in the exact combinations of medium, ground and application that matter to your work. Keep a note of pigment numbers and dates, then compare the exposed and protected halves in neutral light.

Such tests also sharpen observation. They encourage painters to see a colour not only as an immediate sensation, but as a material with a history, a composition and a future.

A lasting palette is built gradually. Let every tube earn its place through the marks it makes, the mixtures it yields and the confidence it gives you when the painting leaves the studio.