I began writing an explanation of color index codes in another post, but it became too long so I’m putting it in its own post!
The short explanation is that watercolor paints have “ingredients” labels that can be read to identify which pigments are in them. For example, Ultramarine Blue has the code PB29 (Pigment Blue #29). If you see PB29 on a paint label – for example, in Daniel Smith’s Undersea Green, which is PB29, PY150, PO48 – you generally know you can mix a similar color with Ultramarine Blue as one of the ingredients.
The long explanation? You can learn to decode these seemingly arbitrary color codes, and here’s how!
What is a pigment?
Pigment is what gives watercolor paint its color. Pigments can come from a natural source (e.g. minerals), or they can be created synthetically in a lab. Pigments are similar to dyes; the difference is that dye is water-soluble, whereas pigment is not. In paint, pigment is suspended in a medium (in watercolor, mediums are things like gum arabic and honey, which hold the paint together).
Color Index Codes
The Color Index was created in 1925 by the Society of Dyers & Colourists and it keeps records of different types of colorants. There are two types of code:
- Color Index Generic Name (CIGN) – Color code that is usually printed on paint tubes. For example: PB29, PY150, PR255. This is what the rest of this post will be about. When I say “color index code,” this is what I mean.
- Color Index Number – Entirely numeric. I don’t know these; they don’t appear on paint tubes so I never see them.
Interpreting Color Index Generic Names
Let’s take a color pigment code and break it into parts. For example, let’s look at PB15:3, which designates Phthalo Blue Green Shade.
| P | B | 15 | :3 |
| Pigment | Blue | #15 | variation 3 |
| First position: Type of substance | Second position: Color or hue | Third position: Index number | Fourth position: Variation (optional) |
First Position: Type of Substance
Indicates whether it’s a pigment, dye, or what. You might see one of these letters:
- P: Pigment. Most watercolor and other commercial paints use pigments, so typically you’ll see codes starting with P.
- B: Dye. Rarely used in watercolor because dyes are not lightfast; usually only used for Opera Pink and similar fluorescent colors
- N: Natural dyes. Sometimes used in paints of historical interest, also generally not lightfast.
Examples:
- PR254 = Pigment Red #254 = Pyrrol Red
- BR1 = Dye Red #1 = red fluorescent dye
- NR9 = Natural Red #9 = Rose madder made from the roots of the madder plant (Rubia tinctorium)
Second Position: Hue
Second letter (or two): Indicates the hue or color along the color wheel. Options:
- R: Red
- O: Orange
- Y: Yellow
- G: Green
- B: Blue
- V: Violet
- Br: Brown
- Bk: Black
- W: White
Examples:
- PR108 = Cadmium Red
- PO20 = Cadmium Orange
- PY35 = Cadmium Yellow
Third Position: Index Number
From my perspective, these numbers seem arbitrary. I think this order is loosely designed to group chemically similar colors together, but I don’t really understand it. I’ve eventually learned the numbers of my favorite pigments by memorization.
Examples:
- PY3 = Hansa Yellow Light
- PY65 = Hansa Yellow Deep
- PY97 = Hansa Yellow Medium
- PY154 = Benzimidazolone Yellow
- PY175 = Benzimidazolone Lemon
Fourth Position: Variant (Optional)
This is an optional position; you don’t always see it. But occasionally there is a colon after the index number with another number, which designates variants of the same color.
Example:
- PB15:1 = Phthalo Blue (Red Shade)
- PB15:3 = Phthalo Blue (Green Shade)
- PB15:6 = Phthalo Blue (another option for Red Shade)
Why use Color Index codes?
Manufacturers tend to give paints poetic, uninformative names, so using the Color Index code is sort of like using the scientific name of a flower instead of the common name: it lets you compare paints made from the same chemicals across different common names.
If you know a paint is made from, say, PB27, even if it’s not called Prussian Blue like PB27 typically is, you know certain things about it:
- It will probably look like other Prussian Blues you know.
- It’s nontoxic.
- It will likely be transparent, non-staining, and share other typical properties with other Prussian Blues.
It’s really informative to know the pigment index code!
Exceptions
Generally, using the Color Index code helps you to identify similar colors, even if they’re named differently. For example, two paints that use PG7 will look similar across brands, even if one is called Phthalo Green and one is called Viridian Hue.
However, some colors can use the same pigment code, and still end up looking different. Sometimes, there are different ways of preparing pigments so that two paints made from the same pigment may look quite different. If the same company sells more than one paint made from the same pigment code, generally there is some difference between them that may not be obvious from the label.
Here are some common offenders:
- PBr7 (Pigment Brown #7, a brown iron oxide) is used to make Raw Sienna, Burnt Sienna, Raw Umber, Burnt Umber, and sometimes other browns (such as Van Dyke Brown). This brown can come in a wide range of colors from nearly yellow, to more orangey, more greenish, or dark brown. The “burnt” colors are usually darker because it becomes darker-colored when heated.
- PR101 (Pigment Red #101, a red iron oxide) is used to make Transparent Red Oxide – which is actually more of an orangey color similar to Burnt Sienna – as well as Indian Red, an opaque earth red-brown, and various other colors that vary quite a bit in their orangeness/redness, mutedness, and opacity.
- PV19 (Pigment Violet #19, a quinacridone red-violet) comes in two major flavors. The alpha form is used to make warm reddish pinks like Quinacridone Rose, or Da Vinci’s pinky crimson, Alizarin Crimson Quinacridone. The beta form is used to make more true violets like Quinacridone Violet. Often, brands will offer both a PV19 Quin Rose and PV19 Quin Violet, and some, like Da Vinci, will often even more subtly-different PV19 colors.
What about paints without a color index code?
Some paints use new or unusual pigments that don’t have color index codes. But sometimes paint manufacturers simply don’t tell you what’s in their paints, even if they use typical, ordinary pigments. They aren’t required to. It’s just good practice (and I personally avoid buying paints if I don’t know what’s in them).
Here are some examples:
- Winsor & Newton’s “Cadmium-Free” colors don’t have color index codes. For example, they sell a “Cadmium-Free Red” which is meant to look like Cadmium Red but be a nontoxic alternative. I can’t say for sure what’s in it, because they don’t tell us, but it sure looks like a pyrrol red.
- Daniel Smith’s Primatek colors have been examined in labs and found to contain typical pigments like Phthalo Green and so on (depending on the color), but they don’t disclose that. The marketing implies they are made entirely from minerals.
- Stuart Semple’s Culture Hustle watercolors do not disclose color indexes but are almost certainly made from standard pigments.
The point being, just because a manufacturer doesn’t disclose the contents of a paint, it doesn’t mean it’s anything special.
More Information
Find any pigment you want at the Art is Creation Pigment Database or Bruce MacEvoy’s handprint.com.
For comparison of different brands’ versions of the colors, Oto Kano’s website and Jane Blundell’s website are great resources.