Hello everyone....
Charles....taggants are a florescent material applied either during the paper making process or to the surface before or after printing that triggers the electric-eye canceling machines. I'll post some photo's of samples later today. (I need to take some pictures)
CTO's are created by postal agencies using stamps that are created especially for this purpose or stamps that are leftovers/outdated by the release of new issues.
My statements above refer to stamps issued for public use that are altered by a collectors' activity. As I stated, in some ways there are acceptable alterations.....getting canceled stamps for your own purposes vs an intention to defraud.
For example using Toledo Browns to create canceled stamps to sell and using new commeratives on personal mail to create a used set for yourself. Here's an example of something that I created by getting a courtesy cancel (having an item canceled and immediately handed back rather than having it sent through the mail).....there are restrictions to this process....but it is legitimate) I created this to document the time period of possession of the color shades.

There are collectors that create many similar items and there are many that frown upon this activity. Neither can be said to be absolutely right or proper. As with many aspects of the hobby it is up to each individual to decide for themselves. I always try to look at an individuals seeming intention, if the items were created singularly or in a set within period use vs if there are dozens of a single item from a later period that would seem to have been created specificly for sale. The first is acceptable, the second is not.
Also, the value of a used Toledo Brown with a cancel from the period of use would be worth much more than one with a recent cancel. One with a recent cancel would have a minimal value of 20-50cents, a period use cancel could carry a value of $5.00-$10.00 now, in the future the recent cancel would stay about same, but the period use cancel could increase by a great deal.
A lot of this type of thing is debatable. If I were to send something to you, that cover would be seen as a collector made item even though it would be just a case of me using the postal system. Collector made items are worth less than just a happenstance of everyday use of the mails.
Dianne..... Almost all stamps have a variety of shades. Many stamps are issued in different formats, sheet, booklet, coil, and imperf. Some have true color varieties.
Color shades usually don't get a separate catalog listing number, color varieties generally do. As do the different formats.
examples of shades...



example of color errors The Bronx Blue is listed as an error but the magenta Douglass is not...
#2466 65cents mint, 20cents used. #2466b $6.00 mint, $4.50 used


#1290 rose lake 55cents mint, used 20cents & #1290b magenta $25.00 mint, no value listed used.