It's the copper coming out of the brass alloy. You can brush it off with a toothbrush. Highly recommended you do not use the one you do your teeth with. Isn't a good idea to borrow somebody's either. Even though that'd be really funny.
The vinegar doesn't have to be high quality. Any vinegar, vinegar being citric acid, will do nicely.
"...Spelling is..." Yeah! snicker.