Much of the commercially advertised “yellow rhodium” is a
questionable thickness of rhodium plate on brass.
But there was a most interesting paper published in 1965 out of Los
Alamos National Laboratory, the only serious paper I have seen on
yellow rhodium. It points out that dirhodium trioxide Rh2O3 is yellow
in color and stable. It is insoluble in water or acids.
Amazingly, there is a scanned copy of this typewritten near-50yo
paper on the web! Find it here:
Radiochemistry of Rhodium
http://www.ganoksin.com/gnkurl/19n [pdf file]
Take a look at page 11 for the above that I remembered.
Take a look at page 9 for oxidization techniques.
I have a strong feeling this has nothing whatsoever to do with the
product you are seeking, but it demonstrates there are chemical
methods to make rhodium show a distinct yellow oxidized surface.