What Do the Symbols on Paint Tubes Mean?

A tube of artist paint carries much more information than the color name. Small squares, plus signs, stars and codes tell you how opaque the paint is, how well it resists fading, which pigments it contains and why it costs more or less than the tube next to it. Once you can read them, choosing paint becomes much easier.

The exact symbols vary from brand to brand, so always check the manufacturer's color chart. The systems below are the ones you will meet most often.

Opacity Symbols: Is the Paint Transparent or Opaque?

Opacity is usually shown with a small square, occasionally a circle. The more of the square that is filled in, the better the paint covers what lies beneath it:

• Empty square: transparent. Light passes through the paint, which makes it ideal for glazing and luminous layers.
• Square filled diagonally by half: semi-transparent.
• Mostly filled square (often with a small empty corner or strip): semi-opaque.
• Fully filled square: opaque. The paint covers underlying layers well.

Some brands use letters instead: T (transparent), ST (semi-transparent), SO (semi-opaque) and O (opaque). Neither end of the scale is better. Transparent colors are ideal for glazes and washes, while opaque colors are best for covering, corrections and flat areas.

Lightfastness Ratings Explained

Lightfastness describes how well a paint keeps its color when exposed to light. A fugitive color can fade or shift noticeably within a few years, so this is one of the most important marks on the tube.

ASTM Lightfastness Categories (I–V)

ASTM International, an American standards organization, publishes test methods and specifications for artists' paints. Colors are placed in categories:

• ASTM I: excellent lightfastness, the highest rating.
• ASTM II: very good lightfastness, still suitable for artists' paints.
• ASTM III: fair. Colors in this category are not considered permanent enough for artist-quality paint.
• ASTM IV and V: poor and very poor.

The Blue Wool Scale

Many European manufacturers use the Blue Wool Scale. A paint sample is exposed to light alongside eight reference strips of dyed blue wool that fade at known rates. The scale runs from 1 (very poor) to 8 (excellent). Ratings of 7 and 8 roughly correspond to ASTM I, and 6 to ASTM II.

Plus Signs and Stars

Brands often translate test results into their own symbols. One widely used scheme marks lightfastness with plus signs: +++ for colors expected to last at least 100 years under museum conditions, ++ for 25 to 100 years and + for 10 to 25 years. Other brands use stars, where more stars mean better lightfastness, or letter codes. The principle is the same, and the brand's color chart explains the exact meaning.

For work you want to last, choose the highest rating available: ASTM I, Blue Wool 7–8, or the top symbol in the brand's own system.

Series Numbers and Price Groups

A series number, often 1 to 5 or a letter such as A to E, shows the price group of a color. It reflects the cost of the pigment, not the quality of the paint. Cadmium and cobalt pigments are expensive to produce and sit in higher series, while earth colors such as ochres and umbers are usually in the lowest. Within one professional range, every series is made to the same standard.

Pigment Codes: What PB29 or PW6 Means

Often the most useful information on the tube is the pigment code from the Colour Index, an international reference system for colorants. The letters identify the pigment family and the number identifies the specific pigment:

• PY pigment yellow, PO orange, PR red, PV violet, PB blue, PG green, PBr brown, PBk black and PW white.
• For example, PB29 is ultramarine blue, PB15 is phthalocyanine (phthalo) blue, PW6 is titanium white and PBk7 is carbon black.

Pigment codes let you compare paints across brands, because color names are not standardized: two paints called “Sap Green” may contain completely different pigments. A paint made from a single pigment usually mixes more cleanly than one made from three or four.

The word hue after a color name, as in “Cadmium Red Hue,” means the traditional pigment has been replaced with a different one, often to reduce cost or avoid a hazardous pigment. Hue colors can be excellent, but the pigment code tells you exactly what you are buying.

Other Marks You May See

• Staining: on watercolors, a symbol may show that a color stains the paper and is hard to lift.
• Granulation: some watercolor ranges mark colors whose particles settle into the paper texture for a grainy effect.
• Binder: oil colors may name the oil used, such as linseed or safflower.
• Safety labels: hazard pictograms and warnings appear when a product contains substances that require them. Read them and follow the safety advice.

FAQ

What does the square on a paint tube mean?

It shows opacity. An empty square means transparent, a half-filled square semi-transparent, a mostly filled square semi-opaque and a fully filled square opaque.

Which lightfastness rating should I choose?

For finished work, choose ASTM I or II (Blue Wool 6–8), and ASTM I for maximum permanence. For sketches and practice, the rating matters less.

Is a higher series number a better paint?

No. The series reflects the pigment cost. A series 1 earth color can be just as permanent and well made as a series 5 cadmium.