I get this type f tarnish on my gold capped constellation. I leave it and some of it comes off as I wear and some stays. Wood produces lots of acidic gases, so a wooden drawer, is often quite good at producing tarnish.
To me the colours produced by the tarnish look like interference colours, like an oil slick on a puddle. Does anyone know for sure? The reason I ask is that someone who is better at physics than me would be able to relate the colour of the tarnish to the thickness of the tarnish layer, and that could possibly be related to the fraction of microns of gold removed during the cleaning.