OK, it's not that I'm a packrat exactly, but how long do you suppose this tube of paint has been kicking around? (For the record, there are also 58 tubes of watercolors...)
I have the terrible habit of buying paints without checking my inventory first. There are paints from my teen years, squatters left by former students, and orphan colors I bought but don’t like. There are also specialty paints, including a few metallics and zinc white.
But most of the tubes of paint in my studio are there because of my carelessness. That’s how I ended up with seven started tubes of titanium white, five different dark reds ranging from alizarin crimson to mars violet, several phthalo green mixes, and many other overlapping pigments.
There is a four-character colour index international (CII) code listed on every tube of good paint. Recognizing pigments from these codes is an important skill for the painter.
Just as Benjamin Moore uses names like “Yukon Sky” to peddle grey paint, art paints are often marketed with evocative names. Generally these names appeal to our sense of tradition, even when the old paint has no relationship to its namesake. For example, Indian Yellow was once made from the urine of cattle which had been fed mango leaves. Today it is made from lightfast diarylide yellow (PY83).
Other obsolete paints are approximated by blends. Naples Yellow started as lead antimoniate, but today is approximated by a blend of four pigments.
Then there are the modern synthetic organic pigments, which I enjoy tremendously. These were developed for industrial purposes and have no historical antecedents. They are great for their high chroma and clarity when tinted with white. The problem comes when they are used to mimic more expensive pigments. For example, I once bought a paint called “viridian” which was not genuine but a blend including a phthalo green. It looked like viridian coming out of the tube but stained like crazy.
When I was sorting today, I found three tubes of cerulean blue. One is Gamblin’s cerulean (PB35), which is “true” cerulean, made of oxides of cobalt & tin. This is a pricey paint but invaluable in the plein air paint-box. The second is cerulean blue hue, which is a much less expensive paint designed to mimic the color and opacity of PB35. It is a mix of zinc white and phthalo blue (PW4, PB15). The third was an off brand which I chucked before noting the contents.
There are places I can substitute the hue for the real thing, but why buy a mix to do it when I already own both the phthalo and white? A good general rule is to stick to single pigment paints whenever possible and mix your own colors. This gives you the greatest latitude.
There are great resources on the web to learn more about pigments. For oils, see Gambin Paint, here. For watercolors, see Bruce MacEvoy’s Handprint.com, here. (I am personally grateful to my friend Kristin Zimmermann for teaching me about CII pigment identification.)