The ink is "fugitive ink" the process of change is "Changeling" I was asking for the process, not ink type. and I saw it listed on a stamp vocab page. IF everyone agrees, though, Tu can take the next question.
What is the philatelic term that matches this definition: An issue released by several postal entities to celebrate a common theme. They may or may not share a keytype design.
Steve you are technically correct, but I was looking for a specific word....I think if you mull over it you will get it.