18K Yellow Gold Oxidation Problem

I had a ring made with sterling and 18K yellow gold. The 18K yellow
gold was inlaid with the silver. I oxidized the piece in liver of
sulpher and put it in the tumbler. The first time I oxidized the
piece, the 18k yellow stayed it’s natural color. When I took the
piece out of the tumbler, the silver was not oxidized to the
color/shade I wanted. So I brought out the liver of sulpher again
and dipped the piece, took it out and brushed it with a soft brush
and some dishwashing liquid. The gold was still ok. I dipped it
again for a couple seconds, pulled it out and the yellow gold was
now a copper red. I tried to tumble it, polish it, etc. - and the
gold is now an unwanted dark rust/copper color that I can’t seem to
get rid of. Can anyone tell me what happened here?