Abundant sulfur compounds in garlic are also responsible for turning garlic green or blue during pickling and cooking.

http://en.wikipedia.org/wiki/Garlic

Abundant sulfur compounds in garlic are also responsible for turning garlic green or blue during pickling and cooking. Under these conditions (i.e. acidity, heat) the sulfur-containing compound alliinase react with common amino acids to make pyrroles, clusters of carbon-nitrogen rings.