Help With Belgian Congo #139 Color Variation

*** Moved by Staff to a more appropriate forum. ***
The 10c is listed as gray brown by Scotts and sepia by my Belgian Ex-colonies catalog. I have several other brown/sepia ones. Then the middle version is an odd green. I added the 15c which is listed as gray in the catalogs.

Just curious what to think about the greenie 10c, I wanted to add some text about it, but not sure what to say. Any ideas what may have happened to it?
Thanks for any help.

edit - added smaller image

the color of the paper in the first stamps tells us it was played with . It could be the sun , chemicals or soaked in dirty water . Something is changing it color .