Why does copper turn silver/rainbow in colour

[mfgs]

What causes copper to turn silver and sort of a rainbow effect? Must be a reaction with something, but what? I’ve just seen it on an unvented cylinders which was fitted about a year ago. What is the cure?

 

[Pickwickpick]

Think its contact with zinc that can make copper go silver coloured, so my guess is its reacted with a brass fitting for some reason. My chemistry knowledge is way too blurry these days to know why, but it might be something to with on of or a combination of: flux/being heated with blowtorch/water chemistry that kick starts the reaction.