I couldn't have answered this for you, but Mr. Alex Fullerton, a winemaker and friend, came to the rescue. I quote him:
Anthocyanins (the main pigment in red wine, red cabbage, blueberries, and many other things) are responsible for both the blue color in the grapes' skins and the purple color of the wine. Anthocyanins have a double bond that is formed at one of two different locations of the molecule depending on which location is more favorable, resulting in the molecule having two very different shapes it can take. In high pH it is more favorable for most of the molecules to twist one way resulting in blue hues, while in low pH it is more favorable for most of the molecules to twist the other way resulting in red/pink hues. The skins on the Monastrell grapes are very alkaline resulting in the blue color while the wine is more acidic (but not extremely acidic, maybe around pH 3.8) causing a purple color (roughly half of the pigment is blue and the other half is red, red + blue = purple). A very acidic Burgundy on the other hand with pH under 3.5 will appear very red.
And, from Wiki:
Anthocyanins (from the Greek words for "flower" and "blue") can appear red, purple, or blue, depending upon the pH. In flowers and fruits, anthocyanins make the color red, blue, or purple, in order to attract pollinating insects (in the case of flowers) or predators (in the case of fruits, which need predators to eat and then disperse the seeds in the fruit).
There is nothing cooler than science.
[Photo attribution: Shutterstock.com]
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