Color Trials, unlike of Color Proofs (that have only the old four ciphers numeration, writing in pencil on the lower right side of the cardboard), show different color codes according to the period in which they are printed. Infact, from 1965 and later, the code identification shows two letters followed by a number of 1, 2 o 3 figures.
To this rule, however, exceptions exist:

OLD CODES A 4-figure number followed by the monogram that denotes the ink manufacturer (Lx, Lc) (blue, orange, green, red, violet, black, brown).
NEW CODES A 1- to 3-figure number preceded by two letters (generally capital) that indicate the color; also in this case it can be present the ink manufacturer's monogram (blue, orange, green, red, violet, black, brown).
EXCEPTIONS The color code can be present as a 5-figure number instead of 4 (often this happen for Monaco Essays) (example 1), or in place of the code the color is identified by the extension; for example "Rose Primaire" (primary pink), "Noir de base" (basic black), "Bleu de Prusse" (blue of Prussia), "Vert Olive" (olive green) (example 2).

In the years prior to about 1946, nearly all the numbers were in the 1to1000range plus a very few in the 2000's. With the introduction of multicolor printing after the War, the number of different shades required and the number of different designs or denominations to be given several distinct colors greatly increased. Accordingly the amount of ink numbers used began to proliferate. From number 1100 up, however, it is clear that blocks of numbers were reserved in advance for the primary hues or mixtures.

Most color codes seen are four digit numbers between1100 and1721, and are classified as follows ( old system):

In addition, a few numbers in the 2000's, 7000's, 8000's, 9000's,11000's and 12000's were used, but so few being reported that one cannot be certain of any pattern. All the examples we have seen of the 2000's (all prior to 1940) were colors used for surface tinting of the paper.
However, the old numbers below1000, have continued to be used up to the 1960's at least, though presumably no new assignments of numbers in that range were made after 1945. Some of the numbers below1000 were, after 1945, prefixed with a 1 or 10 and then incorporated into the new series above 1000.

Numbers on the Color Trials of the issues of 1965 on show a quite different scheme of identification. Each color-hue group is given a prefix as follows (new system):

The numbers in each of these groups start with1, continuing in one case up as far as 32. Again there are gaps in the numbers reported so far, and probably for the same reasons as with the old system. But there are smaller totals of numbers reported in each group than for the hue groups of the old system.
That may be because the new system has been in use for only a few years compared to the old one.
Probably it is also because they are now using mixtures of the basic colors and are obtaining additional colors on the stamps by overprinting.

On the Color Trials of the "painting" stamps of France, Monaco, Andorra, etc., which are printed on six-color presses (T.D. 6) by combined direct-recess (taille douce or T.D.) and offset-recess, there is seen a more complex notation of the color numbers.
For example, on a pane of the France (Yvert N° 1587), painting of "Philip The Good", of 1969, the following notation are found:

The left column headed "Report" is for the offset colors, the right column headed "T.D." (taille douce) is for the recess colors. Probably "H", "M" and "B" stand for top, middle and bottom, and refer to the inverse sequence of overprinting of the colors listed.
The "100gr" and "10gr" indicate the proportions in grams of the two inks that were mixed for the "M" color. Sometimes the units of the parts of these mixtures are given in "k"s.
On other Proofs of this type the "Report" column is often headed "R" or "TL".

A special thanks to Mr. Kenneth R. Thompson for his assistance in the english translation.
© Giorgio Leccese