Enzymes are key players in this reaction, and the extra vinegar and lower temperatures are what allow the enzymes to be released.

An enzyme is a protein that acts as a catalyst in an organism. It's normally found inside cells and doesn't react easily. However, vinegar changes this by creating an acidic environment that leads to increased permeability of the cell, and the enzymes in the cell are released. Then, cold temperatures activate these enzymes. Driven by the enzymes, specific amino acids in garlic react with specific acids to form yellow pigments - eh wait, why is it yellow, where's the promised green color? In fact, the metabolism of the sulfur-containing compounds in garlic in the process produces a certain amount of blue pigment again. Both superimposed, the green color appeared. If the garlic is stored for too long, you may find that it gradually "lost color", turned yellow. This is because the blue pigment is not very stable and will gradually decompose. So, if you store it for too long, the garlic will turn from green to yellow.