Garlic is naturally a soft white but can turn blue or green when exposed to an acidic environment.

Lemon juice and vinegar are two common ingredients that will turn garlic blue if the garlic is exposed to it for a long time.

The acidity causes the reorganization of the molecules in the garlic cloves, creating polypyrroles, molecules that alter garlic color to blue or green.

When fermenting garlic, the lactic acid created by the bacteria is what turns the garlic blue.