The 'Gold' Medallions of the 1910 up era may have been what was called "Gilt', which was a very thin Gold wash, which worked well to prevent Tarnishing of Brass Stampings, Medallions or whatever.
For what then was a $16.00 Ounce of Gold, they could 'Gild' many many thousands of Medallions for a very low cost per unit.
I do not know what process S&W was using, but, just saying.
Nickle Plate over the Brass Stamping would make sense for the later 'Silver' color Medallions...and make more sense than Chrome, or, Chrome would have been an unusual choice at any rate, for the time period.
Now that I am thinking on this, and looking at some critically, the earlier 'Gold' Medallions do have the look of Gilded or Gilt Brass, so, I am leaning to that method, as being what S&W was doing then.