White wines are generally made with grapes with yellow or green skins.  White wines can also be made from black-skinned grapes if the juice is separated from the grape skins early enough-i.e., before fermentation.  Red wines get their color from being fermented in contact with the skins of dark grapes.  Rose’ gets its pink color by either a short contact time with the skins of dark-colored grapes before fermentation or by mixing finished red wine with finished white wine.