Color, insofar as pigments are concerned, is not an exact science. There is much subjectivity owing to manufacturing processes, pigment variations and even the way we ourselves perceive a color.

The Color Index Name (CIN) is an internationally accepted code used to identify colors by pigment content. It consists of the category (either pigment or dye), the general hue family (red, blue, green, etc.) and an assigned number, which is based on chemical or mineral composition. For example, the CIN for Dioxazine (Diox) Purple is PV23:
P = "Pigment" (as opposed to a dye)
V = Violet (hue group)
23 = Assigned number based on chemical constitution, in this case, carbazone dioxazine.

• PBk = Pigment Black
• PR = Pigment Red
• PG = Pigment Green
• PV = Pigment Violet
• PY = Pigment Yellow
• PB = Pigment Blue
• PBr = Pigment Brown
• PO = Pigment Orange
• PW = Pigment White

No matter which medium you choose to paint in, the pigments used to formulate your colors are exactly the same.

You will also find that in some instances the same CIN appears on several seemingly different colors. The different colors are a result of variations and/or modifications of that pigment due to the cooking process, natural impurities specific to a particular mining site or by chemical additions to modify the original pigment. A good example of this is PBr7 (Pigment Brown 7), the designation for the umbers and siennas. This pigment is basically made up of iron oxide and manganese, with trace amounts of naturally occurring minerals. Raw sienna contains an increased amount of iron oxide. Burnt sienna contains an increased amount of iron oxide, plus it is heated. Raw umber contains an increased amount of manganese. Burnt umber contains an increased amount of manganese, plus it is heated.

Often different manufactures give their colors proprietary names. Or sometimes the same, or very similar, names will be used by different manufacturers, but will not be the same color at all. An example of this is ultramarine blue, with the CIN PB29, pretty consistent from manufacturer to manufacturer with one major exception, Jo Sonja's version called "Ultramarine". That's a different animal altogether. The "blue" is omitted in their color name, but it can be confusing to one who's shopping for "ultramarine blue" and is not familiar with CIN's. This color is actually a mixture, consisting of PB29 (yup, the CIN for ultramarine blue), PW6 (the CIN for titanium white) and PB15 (the CIN for phthalo blue). This color behaves very differently if used in mixtures calling for ultramarine blue. The corresponding color for ultramarine blue in the Jo Sonja's line is called "Ultra Blue Deep" and it is "straight" PB29. An interesting (but probably useless) tidbit about ultramarine blue is that it used to be made from pulverized lapis lazuli, making it a very expensive pigment. The ultramarine blue we know today with the CIN of PB29 is technically known as "artificial ultramarine".

Keep in mind that there can be (and often are) variations in the color between brands due to a number of factors: coarseness or fineness of the pigment grind, formulas, origin of pigments and fillers, if any. Sometimes the variations are slight enough not to cause any serious problems in mixes, but be aware that sometimes the variations can be quite extreme. It's usually a good idea to purchase the brands called for, if at all possible. This is especially important for the "earth colors", as these will have the largest variations which will yield far different results when mixed. The earth colors are called this because their pigments are mined and are made up of natural clays and minerals in various quantities. The variations in pigment are cause by geographical location of the mines and variations of the trace minerals. Examples of earth colors are the ochres, umbers and siennas, among others.

Ideally, you should mix your own umbers, siennas and ochres using complimentary colors. I do mix most of my earth tones, and I strongly urge that you learn to mix your own earth tones at some point. There are a number of books available on color theory and mixing. I have made a list of my favorite books that deal with this: My Book Recommendations

Last, there's often another set of letters found used in conjunction with the CIN:

• R.S. = Red Shade
• B.S. = Blue Shade
• G.S. = Green Shade
• Y.S. = Yellow Shade

These designations are primarily seen with the phthalo color CIN's (but not all manufactures make this distinction on their CIN). This specifies color properties of the hue. For example, phthalo blue can have red tendencies (PB15:1 R.S.) Or green tendencies (PB15:4 G.S.). This is helpful to know when using these colors in a mix. Phthalo blue R.S. will yield a less vibrant color if mixed with greens than phthalo blue G.S. due to the conflict of the complimentary colors red and green. Mix two complimentary colors together and, depending on the two colors, muted grayish and brownish colors are the result.