Pigments are chemical substances that give colour to things. If we're looking for a more in-depth definition, we can take a look at this one:
"A pigment is an intensely colored molecule that often lends color to other materials, and is either partially or completely insoluble in water. Pigments are different from dyes, because pigments require a binder (or binding agent) to dissolve into liquids."
So a pigment is a molecule that makes things coloured, got it. Pigments can be of any colour: black, white, grey, yellow, orange, red, pink, turquoise... Any colour you can think of can be achieved by mixing or producing pigments. There are pigments for every single hue you can think of: instead of having a single default blue pigment, hundreds of different blue pigments exist.
But how do you tell them apart if there are so many of them? Is there a way to categorise pigments?
Yes! There's a simple way that's internationally used by artists, art restorers, museums and paint manufacturers.
All pigments are categorised with an alphanumeric code. Many pigments can be referred to with several different names depending on the brand that sells the product, the country where it's been named, etc. For instance, the same shade of yellow is known as Hansa Yellow Light, Hansa Yellow, Lemon Yellow Primary Yellow and Process Yellow (amarillo proceso in Spanish) across different brands and countries. Colour Index International (CII) resolves these conflicting names so that manufacturers and consumers can identify the exact pigment they're looking for; in the case of the aforementioned yellow, it's pigment number is PY3 no matter the name given.
All pigment codes follow this formula:
P[first letter of colour it refers to][numeric code]
For example, PY3 is a code where the P stands for 'pigment' while the Y stands for 'yellow' and that 3 refers to the molecular structure of the pigment.
Not every colour has a pigment code, there's no such a thing as "PP" for pink pigments as pinks are classified as magentas, part of the red pigment family.
These are the codes you will find:
Those are the main nomenclatures you'll find on your watercolour and oil paint tubes. Some paint tubes contain a convenience colour premixed with two or more pigments which means you can achieve that exact same hue by mixing the single pigments yourself. For example, many brands sell a convenience turquoise that contains PG7 and PB15:3, if you already own tubes/pans of standalone PG7 and PB15:3, you could save some money.
We've already established that paying attention to pigment ingredients on paints can help you save money (unless you really want that colour, who am I to stop you).
What else could be a reason why we should know pigments?
Easy: to avoid disappointment and false advertising. Some watercolour brands (both commercial and artisanal) will advertise as lightfast colours that are made with highly fugitive pigments.
Kimberly Crick offers for free an exhaustive list of fugitive pigments:
For a general pigment database, go to artiscreation.com where you can find every single pigment known classified into colour families.