2574.It shouldn't surprise anyone from student level on up that not every red is identical, for example: this cadmium yellow will look a little different and mix a little different than this other one.
As Muse Art and Design has posted here (http://museartanddesign.com/2011/03/paints-pigment-codes/), there's more to the color in your tube of paint than just the artistly name on the front. For instance, I have in my hand a tube of Winsor & Newton Galeria Cad Yellow Medium. If you look on the back of the tube, underneath the color name is the line
Pigment: Cadmium Zinc Sulphide (PY35)
All pigments are made of metals and/or various chemicals. Words like cadmium zinc sulphide, however, may seem too abstruse. But never fear - the code at the end, the PY35, is part of a standard coding system amongst producers of artistic paints.
It's rather simple to learn and not to difficult to understand. The letter P signifies "pigment", naturally, with the next letter signifying the class (R=red, O=Orange, Y=Yellow, G=Green, Br=Brown, Bk=Black, W=White, and M=Metal) of color. The number uniquely identifies the chemical combination that produces the pigment. To get really abstruse about it, the color PY1, or Pigment Yellow 1, can be marketed as Cadmium Yellow Hue, Hansa Yellow G, or Permanent Yellow Deep as well as Permanent Yellow Medium. But it's all PY1, a Monoazo, arylamide, with a CAS registry number of 2512-29-0. On the other hand, PY3 is also a Hansa Yellow, has been marketed as Arylamide Yellow, Bright Yellow Lake, Cadmium Lemon Hue as well as Cadmium Yellow Lemon Hue, but it's all PY3, an Organic, Monoazo, Arylamide, CAS registry number 6846-26-6.
That's quite a prodigious flood of info, I know, but the point is, look on your paint tube and find that pigment code number. For instance, the Cad Yel Med from Winsor & Newton, in both the student and professional ranges, come in PY35. Another manufacturer, Liquitex, has a color in its Basics value-student range called Cadmium Yellow Medium Hue, which is made of PY74 (Arylide Yellow 5Gx) and PY83 (Diarylide Yellow).
Once again, a lot of abstruse info. The point I'm making right now is not that you have to know what a Diarylide anything is, unless you're that interested in chemistry, but each and every pigment mixes a bit differently and behaves a bit differently. If you came to the end of your tube of W&N Galeria Cad Yellow Med and all you have around is a tube of Liquitex BASICS Cad Yellow Med Hue, you'll probably want to do a little mixing off-palette to see how it'll react.
This is also useful info in the way Muse's article expresses, that knowing how pigments mix to create the packaged colors you buy could enable you to create your own mixes in a pinch from what you already have on hand. And knowing how to read pigment codes goes a long way toward enabling you to make consistent paint buying decisions despite the sometimes confusing welter of names colors can go by.
Muse's blog article is here, again http://museartanddesign.com/2011/03/paints-pigment-codes/; and if you want to go really deep into what all these different pigments are, the Color Of Art Pigment Database can be had via http://www.artiscreation.com/Color_index_names.html