Colorful Foods Day 4
Q. Why do the same color grapes produce red wine but purple grape juice?
A. Yes, both are made with a combination of red, purple, and black grapes, but...
As I understand it, for red wine, the grape skins, stems, and seeds are all used. In the skins and seeds of red grapes are compounds called Tannins, which release a red color during the fermenting process. Tannins can also be released from the barrels used to store the aging wine, which can add a bit more redness, too.
Grape juice, however, is mashed and strained, and therefore contains mostly just the juice. Without the extra tannins from the skins, etc, and the barrels, the color ends up more as a blending of the natural grape-color. Plus, most commercial grape juice is made from Concord grapes, which tend to be more bluish-purple in color anyway.
That's my best guess at least. :) I have a very limited wine knowledge, so if you know other reasons, please comment!