I have discovered, much to my chagrin, that many colors in the colored pencil box are somewhat fugitive, that is to say that they are not extremely colorfast. It's an important consideration, since you don't want to sell a piece and then have it fade away. (Those lovely magentas are often the first to disappear!) I have bought boxes of colored pencils, only to find that a quarter of them were not very lightfast. I gave those colors to teachers to use in classes with kids or to kids in the neighborhood.
Here is a resource that you might want to keep on hand, although it is pretty exhaustive and has colored pencils I have never heard of, let alone used:
May your colors not fade away!
Sonja