A few years ago, the Scott US Specialized catalog changed to further define missing ink colors on stamps. The missing ink can come from one of two general categories.
Color omitted stamps occur when the printing plate fails to deliver ink to the paper. The well that holds the ink has gone dry. Or in some cases, the printing plate is disengaged for some reason and some portion of stamps doesn’t receive the ink. These stamps are caused by production related problems.
Color missing stamps occur for one of four other reasons:
FO – “foldover” is when a portion of the paper is torn and folds in half. The ink is transferred to the back of the paper that folded over. The stamps underneath that paper will miss receiving that ink.
EP – “extraneous paper” is a bit of a misnomer. In these cases, a stray piece of paper falls onto the printing press and receives the ink instead of the stamp. These items are easy to identify ususally because there is an obvious unprinted area of the stamp. Yes, in most cases, this is caused by a stray piece of paper. I think a more proper term would be “FM” for foreign matter. A piece of string, a leaf from a tree, or any foreign object could potentially fall onto the sheet of paper and prevent ink from being transferred to the stamps.
“CM” – “color misregistration” a misregistration of the printing plate may result in the ink missing on the stamp
“PS” – “perforation shift” is when the perforations are shifted such that the ink is missing on the stamps.
All of these are errors. But to the collector of error stamps, color omitted stamps are preferred and in general, more valuable because they are errors created through the printing process itself and not an unusual set of circumstances.
The FO, EP, CM, and PS items are usually easy to identify because you can see where the paper was folded over or where the perforations were shifted.
Suppose a single error stamp appears that has a missing ink color. If the perforations are normal, it can’t be a PS. But is it a color omitted stamp versus a FO, EP, or CM color missing error? Luckily, when collectors find errors like this, they usually save the entire pane of stamps (or whatever the multiple is) to show what caused the error.
In the absence of any other evidence, a single error stamp would probably be noted as a “color omitted” error by the catalog editors.
Value is determined by demand and supply. The fewer the number of errors there are and the more demand there is makes for a higher price.