Tsuyoshi,Originally Posted by Shinnya
You can check the manufacturer's literature, and you can look on the back of the tubes as well, but that can be somewhat hard at times, and not all the manufacturers have decent literature.
Also, gum printing is somewhat of a crossover process, and the knowledge of paint is truely outside the realm of knowledge most photogrphers have. To bone up on this, I recommend this site:
There's more there than you can possibly imagine. That site lists paints by color and brand, and indicates the makeup of the paints as well. There's also color theory, paint background information, and much, much more. It's a great reference and starting point for learning about the pigments used in the paints, and I recommend everyone interested in the process to do some reading on that site.