General

Thank you all for the information!

Luis, Oxidation as mentioned by others is certainly the culprit. However, these color changelings can certain add to your collection. - Glen

Definitely oxidation.

Hi. I believe that the brownish color of the three stamps shown are the result of the oxidation of the yellow ink. They are not a printing variety.

Hello all, among my collection I have this 20 cent stamp of 1881 SC 25. The really interesting thing about this one is that it has a very noticeable color variation, I would dare to say that it is the same color as the 2-cent stamp of that same series, I also have other 20-cent stamps with color variation but it is always a darker or lighter tone of the original color of the stamp (like orange), but nothing that looks like this one. It is important to mention that it is a new stamps