pH Sensitivity of Purple Grape Juice

I have blogged before about grape juice as a dye (see: http://campbelldemo.blogspot.com/2012/05/grape-juice-prints-on-clothes.html), blackberry juice as a dye (http://campbelldemo.blogspot.com/2012/05/dyeing-seashells-with-blackberry-juice.html) and pigments in plants being sensitive to pH (http://campbelldemo.blogspot.com/2012/05/ph-sensitivity-of-colorants-in-flower.html).

So, I think it is time to mention an effect that I have been noticing for years but saw spectacularly this morning.  Purple grape juice is red out of the bottle (or out of a frozen concentrate can), but when the juice is diluted the color shifts to blue.  I interpret this as the anthocyanin pigments changing color when the pH of the solution increases as tap water is added to the juice.  The picture shows the reddish undiluted grape juice at the left, and grape juice blue as it is diluted into water at the right.