In ancient times artists made their own paints, so they knew exactly what to expect from them. As the paint industry began to develop with the discovery of new pigments and synthetic colors during the nineteenth century, this practice fell by the wayside. In recent years, however, artists have taken a much greater interest in their paints, partly because of a greater awareness of the dangers inherent in some pigments.
Because of the efforts of many in the industry the American Society for Testing and Materials (ASTM) has established a set of voluntary standards for paint manufacturing, which includes labeling of the products to include important information for artists. I recommend that artists learn the meaning of the terms on the labels so they can buy their paints to suit their needs.

The information will vary depending on the medium and the manufacturer. Here are some significant numbers and symbols on my Winsor & Newton artists' watercolors:

• Paint name, given in five languages, followed by manufacturer's stock number
• Series number, designating the price level from 1-4 with 1 as lowest price
• Permanency rating AA, A, B, with AA highest and B lowest (moderately durable, 3 colors only in W&N) N/L means Not Listed--most of the newer colors haven't as yet been rated by ASTM, although manufacturers claim they have been tested by ASTM standards during manufacture.
• Lightfastness, the ASTM rating shown on newer labels; I=excellent, II=very good, III=moderate, IV=poor
• Weight--given in milliliters and fluid ounces
• Health label--ACMI (Art and Craft Materials Institute) rating for safety of use
• Pigment name and Color Index Number--an international classification designating the chemical composition and a code number for the pigments used in the paint, e.g. PB15:3=phthalocyanine blue green shade

All of this information is important, but artists have different criteria for paint selection. One might be more interested in toxicity, another in lightfastness and another in size of tube and price level. Not all manufacturers have this much information on their tubes, but you can usually find what you want to know in their sales literature, by calling customer support or checking their web sites.

See my other paint articles and chapter three of Exploring Color pp. 32-45. Also, look at my paint FAQ.

You'll find a handy full-color Color Index/ASTM chart in the appendix of my book Confident Color on pp. 152-153.

I highly recomment David Pyle's book, Paint & Colors: What Every Artist Needs to Know, comprehensive coverage from the history of pigments to the behavior of paints and safety issues.