Think of how sweet a raisin is. Sucking all the water out of a grape will lower the degree to which sugar in the thing is diluted, and will jack up the brix level. Table wine is often about 22 brix, while dessert wines will have a much higher brix level. So understandably, vintners get very nervous about the weather close to harvest time. When the grapes are harvested, they are pressed into juice.
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Once the juice has been extracted, the vintner then determines whether to make a batch of red or white wine. Since the juice of almost all grapes is the amber hue of white wine, a winemaker must consciously decide to make a batch of red wine. He or she does so by allowing the crushed grape pulp to sit together with the stems, skins, and seeds of red grapes. This process of maceration obviously adds the dark color to the juice, but at the same time, it imbues the must with tannin from the woody elements of the grape plant. Tannin is what gives red wine is distinctive flavor and what allows red wine, far more than white, to age gracefully over time.