While we are on the topic of garlic, I was presented with a complaint once where the lady had mixed chopped garlic with lemon juice and microwaved it.
It turned bright green.
Some months later I was shown a bottle of garlic cloves, preserved in vinegar, that had similarly gone bright green.
What's going on?
Part of the problem is that this is not how garlic was traditionally treated. It would normally be cooked in a neutral sauce. And a thick sauce that hid it from view.
The two complainants were treating garlic in a a totally different way to normal.
But the reactions were perfectly normal.
Garlic has natural levels of iron and natural levels of sulphur compounds. Heated in the presence of an acid (lemon juice or vinegar) these compounds react to form iron sulphide which shows as a green colour.