What’re you using for patina? That may have something to do with it. If you use liver of sulphur, or something that just doesn’t effect gold, you should be good.
I did some pieces a while ago that were Kum-Boo on cast silver that looked like slate. So the silver was black enough that it really did look like slate, and the gold stood out clearly.
As I recall, I used Rio’s black max to do the silver. Applied with a brush, carefully, so I didn’t get the gold, because black max will do gold. I remember getting some on the gold, and cleaning it up with a toothbrush full of pumice powder, and a little bit of tape to mask off the edge of the silver.
The other trick is the gold foil. I roll my own, so mine’s much thicker than the commercial foil. (2-3x thicker than the normal foil. About .0008-.001" thick.) The thicker it is, the less the silver underneath shows through, which makes it pop more.