Palladium is not a good candidate for showing a significant tonal change from silver. Rhodium or Iridium seem to me to be better candidates, although I have no idea if these will be archival.
Another way could be to go via FSA toning, to get much smaller silver graing before toning. I think this could significantly change the results.

For pure tonal change iron and copper are well known; add titanium, vanadium and cobalt and you should have enough to play with.