A tube labelled Ultramarine Blue can look reassuringly familiar, but the name alone tells you very little about what is inside. The pigment index number guide is the more dependable place to start: it identifies the actual colourant used in the paint, helping you assess a colour’s mixing behaviour, permanence and relationship to another tube on your palette.
For painters who want deliberate control rather than a collection of near-duplicates, learning to read these codes is one of the most useful studio habits. It turns the label from marketing shorthand into practical material information.
What is a pigment index number?
A pigment index number is a standardised Colour Index designation assigned to a pigment. It normally begins with letters that indicate the colour family, followed by a number identifying the specific pigment. PB29, for example, is Pigment Blue 29, the pigment commonly known as Ultramarine Blue. PR108 is Pigment Red 108, Cadmium Red, while PY35 denotes Cadmium Yellow.
These codes are not proprietary paint names. They provide a shared technical language across manufacturers, media and countries. When two paints carry the same pigment index number, they contain the same classified pigment, even when their published colour names differ.
The prefix is useful shorthand:
- PB means Pigment Blue
- PR means Pigment Red
- PY means Pigment Yellow
- PG means Pigment Green
- PV means Pigment Violet
- PO means Pigment Orange
- PBr means Pigment Brown
- PBk means Pigment Black
- PW means Pigment White
The number after the prefix matters most. PB15 and PB29 are both blue pigments, yet their character could scarcely be more different. PB15, Phthalo Blue, is intensely staining and often very high in tinting strength. PB29, Ultramarine Blue, is softer, granular in watercolour and valued for its warm, red-biased blue note.
How to read a paint label using pigment index numbers
Look for the pigment declaration, usually printed on the tube, pan wrapper or product information. A single code, such as PV19, indicates a single-pigment colour. Several codes, such as PB15:3, PG7 and PY3, indicate a mixture.
Neither is automatically better. A single-pigment paint gives you a clear, predictable starting point for mixing and is often especially valuable when building a restrained palette. A mixture may be precisely the colour or handling quality you need, particularly for landscape work, quick palette organisation or a distinctive convenience hue. The useful distinction is simply whether you know which you are choosing.
A tube named Permanent Green Light may contain a blue-green phthalocyanine pigment combined with a bright yellow. Another brand’s paint of the same name may use entirely different pigments. Conversely, two paints with different names may share the same pigment code and be close functional alternatives. This is why the index number should guide comparison before the colour name does.
A suffix can matter
Some designations include a suffix, as in PB15:1, PB15:3 or PB15:4. These identify distinct forms or shades within the same pigment family. Phthalo Blue PB15:3 is generally a green shade, whereas PB15:1 is a red shade. For a painter selecting a cool mixing blue or seeking a particular turquoise mixture, that detail is significant.
Do not assume the same prefix means similar results. PR101, an iron oxide red, is earthy, opaque to semi-opaque and moderate in strength. PR122, Quinacridone Magenta, is transparent, powerful and suited to clean violet and bright pink mixtures. The code is a precise identification system, not merely a colour category.
What pigment index numbers can tell you in the studio
The immediate benefit is better palette editing. If three tubes all contain PB15:3 as their principal blue, you may have three useful formulations, but you may also have unnecessary overlap. Compare their additional pigments, transparency, tinting strength and the role each plays in your work before replacing one with another.
Pigment information is equally helpful when a familiar colour is unavailable. If you know your preferred blue is PB29, you can seek the same pigment rather than relying on a name such as French Ultramarine or Deep Ultramarine. There may still be differences in particle size, pigment concentration, vehicle and final shade, but you have made an informed comparison rather than a speculative one.
For mixing, the number can explain a result that initially seems puzzling. A yellow labelled PY150, Nickel Azo Yellow, is transparent and often deep in mass tone, producing luminous greens and rich darks. A yellow labelled PY35 is more opaque and behaves differently when lightened. Both may appear yellow in a colour chart, but their interaction with blue, white and glazing layers will not be interchangeable.
In watercolour, pigment identity also gives useful clues to granulation and staining, though it does not settle the matter alone. Ultramarine PB29 commonly granulates because of its pigment particles, while phthalo colours tend to stain strongly. The chosen gum arabic solution, pigment dispersion and concentration all affect the wash, so the painted swatch remains the final authority.
What a pigment index number does not tell you
A code is powerful, but it is not a complete specification. It does not tell you the proportion of each pigment in a mixture, the precise particle size, the amount of pigment in the formulation, or the qualities of the binder and additives. Two oil colours made with PR122 can differ noticeably in body, undertone, transparency and tinting strength.
Nor does a pigment number replace published permanence information. Lightfastness is a property worth checking on the individual paint, particularly for mixtures and for colours likely to be used in delicate tints or exposed passages. Opacity, transparency, toxicity and drying behaviour also require their own label information.
In oils, the pigment itself is only part of the drying question. Earth colours such as PBr7 and PR101 often dry readily, while some organic pigments may be slower in oil. The particular oil, milling and formulation influence the paint’s feel and drying rate. Treat pigment codes as one piece of evidence, then consider the full technical description and your own tests.
Single-pigment colours and mixtures: choosing with purpose
A single-pigment palette is not a rule of artistic virtue. It is a useful discipline when you want mixtures that remain intelligible. With a carefully chosen warm and cool version of each primary, plus earths and a white, you can learn exactly where a grey, green or violet has come from and repeat it with confidence.
Mixtures earn their place when they solve a real pictorial problem. A pre-mixed neutral may help you establish the quiet structure of a portrait; a mixed green may suit repeated foliage passages; a rich black mixture may give a particular temperature unattainable from a single black pigment. What matters is that the mixture is identified and its behaviour tested rather than assumed.
When comparing apparent equivalents, begin with the pigment list. Then make a small chart showing masstone, undertone, a white tint, a dilute wash or lean stroke, and a few mixtures you use often. This is more revealing than judging colour from a screen or a wet dab on the palette.
A practical way to use this pigment index number guide
Keep a record of the codes in your working palette, alongside notes on opacity, staining, granulation, drying and the mixtures you rely on. When buying a replacement, check whether the pigment designation is unchanged. If a favourite colour has been reformulated, make a test before committing it to an important painting.
It is also worth separating the question, “Is this the same pigment?” from “Will this perform the same way?” The first can often be answered from the label. The second depends on formulation, medium and your particular method of painting. Michael Harding publishes pigment-led information because artists deserve both parts of that decision.
The next time a tube attracts you with a familiar name, turn it over before placing it in your basket. A few letters and numbers can reveal whether it extends your palette, duplicates a colour you already own, or opens a genuinely useful new route through your painting.