Michael is right... some gold chips turn silver. However, not all silver hot stamps started out gold. I believe there are a number of varieties that aren't recognized in TCR of gold vs silver hotstamp. The ones that turn don't turn all at once; you'll see part of the stamp gold, part silver.

I've seen some chips that are listed as GHS in The Chip Rack that are unused and are a fully bright silver in color. I believe these are a different chip and can be collected as a real variety.