Are you sure that it is silver
toning?
I ask this based on my experience with this coin https://www.forumancientcoins.com/gallery/displayimage.php?pos=-60566 It was described as silver and is only documented in that metal. However, I suspected that it might be an over strike of an earlier Punic
electrum issue. l tested the edge by lightly rubbing a silver cloth against a small
area of the coin edge. This would remove any tone. Surprise... the gold color did not disappear indicating that it was inherent in the metal of the coin rather than limited to surface tone. This suggested a mix of gold and silver i.e.
electrum. I think I have tracked down the earlier Punic
denomination which would be the base of the over strike.
Like your coin, my Punic coin changes intensity of gold color dependent on lighting conditions. This reflects the fact that coin is dominantly silver and the gold content is probably around 12-15%. The coin on which I believe it was over struck attracted the comment from
Sear that "The gold content of these coins is so low that they can be easily mistaken for silver."
I know nothing of the
Parthian series, so I cannot comment directly on this possibility for your coin. However, the coin looks to be in low grade
electrum rather than silver as the "gold tone" is uniformly present and consistent, suggesting it may be metal content rather than tone that is giving the gold color. Test a small
area of the coin edge by cleaning as indicated to test wether it is superficial gold color (tone) or in the metal itself.