What Causes The Rainbow Color On Silver Coins?

very informative

The comments of Conder and Sap taken together, give the whole picture exactly.

In the light of these comments, it must be remembered that beautiful rainbow toning is in a very delicate and unstable state between blast white and full dull gray.
To preserve the nice coloring it is essential that the coin is kept in an airtight capsule. If that is not the case now, then take action immediately.

A slabbed coin is somewhat protected in it's prison, but slabs are not airtight. Over the long term, the rainbow toning may still be at risk.