So far as I know, Thompson Enamel is the only company with an intelligently thought out stock numbering system. It might not seem so at first glance, but crack the code and you'll never be guessing at your colors again.
For the purposes of this article I'm going to focus only on Thompson's 1000 and 2000 series enamels for Copper, Silver and Gold; however, the same system applies to their other lines of enamel.
Each color is assigned a four digit number, and each digit carries with it specific information.
___ ___ ___ ___
A B C D
Position A identifies if the color is opaque or transparent. Any color starting with a "1" is opaque. Any color starting with a "2" is transparent. There's a neat little mnemonic you can use to remember this. The words "Opaque" and "One" both start with "O". The words "Transparent" and "Two" both start with "T". The only exception is stock numbers starting with “2” and ending in "00" or "01" which are opalescent rather than transparent.
Position B identifies the color family. For example "3" denotes green. That means every color starting with 23 is going to be a transparent green. This can be very helpful when putting together shading sets. You can see at a glance if they are complementary or not.
Here's a handy list of the color families for you.
Opaque
0 - White
1 - Brown
2 - Beige / Light Yellow
3 - Green
4 - Blue Green
5 - Green Blue
6 - Blue
7 - Pink / Purple
8 - Yellow / Orange / Red
9 - Gray / BlackTransparent
0 - Clear
1 - Beige / Brown
2 - Yellow / Green Yellow
3 - Green
4 - Blue Green
5 - Green Blue
6 - Blue
7 - Purple
8 - Pink / Red / Orange
9 - Gray
Positions C and D should be treated as a single two-digit number and denote color intensity. The lower the number, the lighter the color. The higher the number, the darker the color.
With this information, I can take one look at 2680 Prussian, and know immediately that it's Transparent (2) Blue (6) and very dark (80). 1410 Robin’s Egg is Opaque (1), Blue Green (4) and very pale (10). 2600 is Opalescent Blue because it starts with a 2, is in color family 6 and ends with 00.