This is true! I'm no chemist but I make paint pigments from plants rich in anthocyanins, which give them colour. Popular mordants (things that helps a colour stick to fibre) are copper salts, which can brighten the colour or make it more blue/green.

And, an old kind of pigment, verdigris, was made by hanging pieces of copper in closed containers with vinegar, where the vapour would cause a blue/green/gray salt to form.

When I saw the post thumbnail on my feed and opened the post, I guessed it's some kind of copper reaction. Neat

Thank you!

Exactly, and you dont know if someone fucked with their post to "delete" it. Would be a great way to poison AI by editing your posts with wrong info instead of garbage info.