Received A 2017 "99.99% Pure Silver Coin" That Appears Gold On One Side. Any Idea Why It Looks Like?

It is a mystery and without knowing the chemistry of the wash used by the RCM, everything else can only be conjecture. It is a bit disappointing that the RCM is silent on this but I can imagine if this is widespread, they would be open to a fairly significant financial impact if they were forced to recall these tarnished coins.

As to making your money back... Not sure on this either... the coins at my LCS are dust collectors... there is no elevated demand for these coins.

So this set kind of leads to a bad batch of wash or cleaner the RCM applied to the finish coins

Not always! See canadian_coins story: (Same as link above.)

- Group of Seven - Golden Coin?

The biggest problem I have with this is that it is clearly some toning/tarnishing type of effect. If it were traditional silver toning, where you get the rainbow effect through to a dirty grey then that would be seen as undesirable in a NCLT proof release at least at the age of these coins (<10 yrs). There are coins that are sold by 3rd parties with "Post Mint Processing" to add colours, plating etc. While they are sold with a premium initially, I haven't seen them retain that premium and most folks classify these as PMD (Post Mint Damage)... in many respects, no different from putting the coin in a bezel to make a pendant.

NCLT Proof coins should be processed to retain the "as minted" lustre, sheen and colour for a long time and if you take care of them, don't subject them to extremes of humidity or environmental pollutants, they should remain in their "as minted" condition... I have many coins that range in age over the last 50 yrs and even a proof 1902 UK crown that still retains the original look.

There has been significant conjecture on what caused this... is it light exposure, is it environmental chemicals, is it humidity, is it temperature... I don't think anyone knows for sure as the results are not consistent across multiple coins... even when stored together... maybe some combination of all but the root seems to be an unfortunate choice of final wash/processing at the RCM prior to shipping. The RCM will never fess up and while the bullion milk spot issue is forgivable... it is bullion after all and milk spots don't impact bullion value, this is just unacceptable for NCLT.

The other concern I have is that these items may be sold as gold plated or worse, a wrong planchet gold issue which would be incorrect... I have no doubt that a XRF analysis will show nothing but silver but outside of an XRF, there is no real way to know without a deeper knowledge of what the initial mint release was or a destructive test on the coin itself. (of course incorrect gold planchet is easy to determine based on weight but there are always those out there who would dupe the inexperienced).

I had the same problem with Golden Maple leaf 1 oz silver 2020. It turned gold on one side after ab 6 months.

Gold toning in silver coin, would the toning turns worse in some way or just keep being golden in colour?
I've seen a recent bidding for a two coin set, one is obviously a gold toning silver coins.
And personally find it looks good in appearance and becomes a rare item as well, since not all of the batch would have that toning effect.
So if it will just stay golden without any bad side effects, I would say, it is not completely a bad thing.

It's been a few months since I looked at my coin. The good news is that the golden toning doesn't fade away...




In case some didn't read my original post, here is the original coin in the set I purchased from the RCM:



This set never saw the light of day except in rare occasions.

Wow, that Group of 7 coin seems to have taken on an even darker golden tone.... as for the post office - my post office has 'sell offs' once in a while for coins that have been on display. Mostly however, any changes is in the packaging from UV damage. I haven't seen the 'golden' affect on their coins.