If you have not changed your silver solder or flux compound there are
several possilbilities for causing your solder seams to turn green.
One is incomplete de-fluxing after soldering. Another is a reaction
between what you are using in the tumbler, perhaps a soap or
brightener and the metal. Ammonia can often cause sterling and other
copper/silver alloys to turn green, unless very carefully washed. Try
making sure the parts are very well rinsed before drying. Tumbling in
cob or shell to dry after rinsing might speed
up the drying process.