Patina-tiny experiments

For The Husband's anniversary art, I've been trying out some green patina. I'll upload a few pix later. In the meantime, this is a little real-world patina I walk past most every day: Rusted and patina'd rebar next to an empty flowerpot.

I'm sure if I thought hard enough about it, I'd be able to figure out why some of the rebar is more green than the rest.

Rust and patina experts: any ideas?