The colour is one of the most easily recognisable characteristics of wine. Colour is also an element in wine tasting since more full-bodied wines generally have a deeper colour.
The colour of the wine mainly depends on the colour of the skin of the grape variety, since the pigments are localised there and not in the juice. All grapes start out as green and the red varieties change hue during veraison in the summer.
The colour of the wine also depends on the vinification and the time the must is in contact with the skins during maceration.
Red grapes can actually produce white wine if they are quickly pressed and the juice not allowed to be in contact with the skins (think Blanc de Noir Champagnes). The colour is mainly due to plant pigments, notably phenolic compounds (athocyanidcins, tannins, etc.)
A wine's colour is altered with ageing by reactions between different active molecules present in the wine. These reactions generally give rise to a browning of the wine, leading from red to a more tawny colour in red wines and from a more greenish yellow to a more golden/brownish yellow in whites. Oak ageing also affects the colour of the wine.