Copper sulfate used to be added to canned peas because it turns green when it oxidizes, making them look greener.

Copper sulphate is straight up poisonous, enough will kill a passion and low amounts will hurt them.

Anyone who wants to learn more about this history, there is a great episode of the “ridiculous history” podcast that goes into the story that finally got food regulations in the US. A team of people who volunteered to be poisoned to help prove that certain things are unsafe to put in food.