Red, or more properly black, grapes, are so named because of the pigment in their skin. The flesh of the grapes produces white juice, and the juice only becomes red when left to macerate on the skins, allowing the pigmentation to leach out.
A very few red grapes, known as teinturiers, have red-coloured flesh. And these can yield dark red-coloured juice, but these are rare and usually not vitis vinifera.
In addition to colour, the skins of red grapes are generally high in tannins, a compound that helps to preserve wine.