Why does garlic sometimes change color?

[Baconologist]

Why does garlic sometimes take on a blueish, greenish or grayish hue?

"Two recent reports from the House Foods Corporation in Japan detail exactly how the garlic and garlic-onion pigments develop. Their creators are the same handful of sulfur compounds and enzymes that give the allium family its unique pungent flavors. Under the right conditions these chemicals react with each other and with common amino acids to make pyrroles, clusters of carbon-nitrogen rings. These rings can be linked together into multipyrrole molecules.

The ring structures absorb particular wavelengths of light, and thus appear colored. The two-pyrrole molecule looks red, the three-pyrrole molecule looks blue and the four-pyrrole molecule looks green, as does its cousin tetrapyrrole, the chlorophyll molecule. Like chlorophyll, all the pyrrole pigments are perfectly safe to eat.

A mixture of onion and garlic favors a blue hue. All the pigments result from a combination of enzyme activity and simple chemical reactions, so you get the most intense color by puréeing the garlic and onion to mix enzymes thoroughly with their targets, then holding the purée on low heat to speed the enzymes without denaturing them, and finally heating it to a simmer to speed the nonenzymatic reactions.

The tint can be muted through higher heat and faster cooking times, a procedure favored by Ms. Jaffrey, who is as unnerved as anyone when the onions in her pan turn turquoise. "I can`t bear that greenish color," she said. "So I quickly sauté the paste to give it a more acceptable pinkish-brown hue."

Source: http://www.nytimes.com/2006/12/06/dinin ... .html?_r=0