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Contrary to popular belief, red wine grapes are not red throughout the entire growing season. Both red wine grapes and white wine grapes are green when they first begin to form. In fact, like all fruits, white and red wine grapes begin as flowers. The first stage of grape growth is the bud burst — this is when a small green growth appears on the vine of the grape plant in spring. The bud burst develops leaves and a collection of flowers called an inflorescence. The flowers bloom small and white, and as they are fertilized, begin to develop into grapes. At this point the natural sugars and pigmentations in the skin have not yet formed.
It is during the ripening of both white and red wine grapes when the sugars and colors are gradually expressed. As white wine grapes mature, they may take on a golden color or, in the case of the Gewürztraminer wine grape, a pinkish hue. They may develop brown specks or splotches on the skin, or they may retain the bright green color of their youth. Some white wine grapes, such as the pinot gris/grigio grape, develop a light reddish purple hue like a red wine grape.
True red wine grapes, though they may resemble white wine grapes during the ripening process, are distinctly darker at maturation. They range in color from deep red, to purple, to almost black. Many develop a dry, dusty appearance on the skin, like a film that can be rubbed off to reveal the glossy skin beneath. Even in a mature bunch of red wine grapes, there may be a handful of green grapes that failed to change color as they grew.
The various pigments that lend wine grapes their colors exist in the skins. The inner fruit of the grape is faintly golden yellow, almost without color. It is during the fermentation process that the color of the wine is decided. The juice from crushed white wine grapes is separated from the skins for fermentation. Juice from crushed red wine grapes, however, is fermented along with the skins, giving red wine its color.
Rosé, white zinfandel, and blush wines have a light pink color because the skin is left with the juice only for a short time period during fermentation. Though this is the traditional method of making pink wines, it is now often forgone. Wine makers will more often blend finished red and white wines to create a pink wine.