[dfariswheel]

A good jewelery store that actually does work on-site can gold plate the medallions for you.

You will have to remove them and that requires care to prevent splitting the wood.
If you're having Swamprat do the work, I assume he removes the medallions for re-working the grips or probably can remove them.

I'd remove them or have him remove them and find a good local jewelery store or metalsmith shop.
If Swamprat doesn't/won't remove them post again and I'll give you instructions on how to do it without damaging the medallions or grips.

 

[dfariswheel]

As above, you want to look around for a jewelry store that does ring work on-site. In order to do setting and other work they need to have a plating setup to do gold and rhodium plating.
Also look for a metalsmith who does fine metal work on other types of jewelery.

Getting a "flash" gold plate on a set of medallions should not cost a lot.

 

[dfariswheel]

I suspect the metal may have changed over the years, but it's some sort of base metal.
"Base metal" is a jewelers term for a metal that's not a precious metal like gold or silver. It's a "baser" or lesser metal.
The medallions are a dull silvery metal, but not soft like zinc or aluminum, nor is it magnetic.
It's not a brass alloy, so that leaves a big range of what it could be.
Sorry, I'm no metallurgist.

I have no idea what Colt plated them with, but it's not real gold. I would think it's some sort of gold colored metal that doesn't easily tarnish, something like the "gold" plating on Marlin 39-A triggers.
Earlier medallions from the 50's and 60's have a more true gold color. Later medallions from the 80's on have a more shiny, "brassy" gold color that looks like something other then gold.

 

[dfariswheel]

The medallions tarnished because they weren't plated with a non-tarnishing metal like real gold.

As above, I don't know what the maker of the Colt medallions used to plate them, but it doesn't look like real gold, but when new they were too shiny for that.

As above, about all you can do is to clean them off as best you can, remove them and replace them with new medallions, or remove them and have a jeweler plate them with real gold.

If you remove them you have to be very careful not to split or crack the wood.
The trick is to use small pliers to unbend the staking on the back side as much as possible, then put them on a firm rubber or soft plastic pad with a hole big enough to pass the medallion then use a punch to tap them out of the hole in the grips.