I wonder  people are using pigments instead of dyestuffs in printing stamp?

There are many  aspects in uv-resistance behavior  between these two kinds of chemicals for sure.

COINTW兄,

This is one interesting subject. Above is one of my favorite reference books which were published by APS. It talks about how stamp is designed and all the way to the final product - all 862 pages long. That's why I call it a "reference book". Chapter 13 is about ink and color. If you're interested, you may be able to borrow one from a local library. If you have one, chapter 13 may bring some facts to light.
Dye and pigment are defined as the similar thing in this book but I would say pigment is one kind of dyes.; the dry substance (pigment) is the coloring matter and the "medium" or "vehicle" is the liquid that holds that coloring matter.
There are two variables to start with - one is inherited by the ink itself (color proofing and shades for instance), the other is the environment which is what this case is.  unfortunately even this book does not discuss in detail about how UV would affect the color on modern stamps. I guess it really depends on what pigment and other chemicals are used. Some "coatings" may be able to prolong the color of a stamp as well.

This book, although published in 1990, covers the manufacturing process of a stamps mostly before 1970. Still it's a good reference book.

I'm just throwing in my two cents.

Understood and thanks a lot.

Frankly ,Although I'm a chemical engineer , I'm not familiar with these chemicals' details at all .

We called pigment 顏料 and dye/dyestuff 染料 in my career , and their usages are

not the same , the sunlight fastness of pigments is lower.

I just googled the web. It seems to me that yours is more appropriate. I learn something new each day.

  A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption.
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber.
Both dyes and pigments appear to be colored because they absorb some wavelengths of light more than others. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments.

There are many other ways to alter the color of a stamp "professionally" ( I hate to use that word here), which I prefer not to discuss here. Ironically, there are people who not only like to collect such kind of stamps, they would even pay a high price for it - this is very different from counterfeit money.  

Green color arrival cancel with year inverted.