The suggestion to gring out the color contrast between gold and
platinum by long, gentle heating made me think–I have no experience
with this situation, but that doesn’t deter me in the least–maybe you
could increase the contrast by depletion gilding, to improve the color
of the gold. IF it wouldn’t affect the plat, which I can’t say. I’ve
had the most luck in enriching gold surfaces by heating and pickling,
but alternating the pickle between regular sparex (to remove copper)
and nitric acid (to remove silver). Someone else may
have a better way. Good luck! --Noel