6 cent air mail

[pdv006]

Good morning all. While going thru air mail.stamps I noticed two brown 6 cent ones. The normal color is orange. Any help is great

[Butterfly]

Browning of old orange stamps is quite common.  People usually say It is the result of exposure to humidity and traces of sulfur in the atmosphere.  Soaking in water and then hydrogen peroxide usually removes most, but not all of it.

The discoloration process  is commonly called sulfuration or sulfidation.

EDIT:  Actually I suspect a bit of bleaching took place as well, since the browning is usually darker than  seen in your case.