You can get a lot of information from looking at a wine in a glass. The colour of wine can tell you about; the grape varietal used, if oak was used in the winemaking process, and even give you a clue about how old it is.

There are colouring agents in the skins of the grapes called anthocyanins. When the grapes are picked and brought to the winery they are pressed. On pressing the grape skins burst and the juice from the grape pulp runs free. White wines will, usually, be separated from their skins straight away, though red wines will not. Red wines initially look just like white wines (a sort of murky green colour) but gain their colour from sitting with their skins for a period of time. This process is called maceration. In this process the skins will impart their colour into the juice. The intensity of colour depends on how long the skins are kept in contact with the juice. 

Roses will often gain their light colour from only being in contact with their red skins for a number of hours, whereas red wines will spend a few days.