Thearubigins are polymeric polyphenols that are formed during the enzymatic oxidation (called fermentation by the tea trade) of tea leaves. Thearubigins are red in colour. Therefore a black (fully oxidized) tea gives a reddish liquor while a green or white tea gives a much clearer one. The colour of a black tea, however, is affected by many other factors as well, such as the amount of theaflavins, another oxidized form of polyphenols.
For a more comprehensive treatment, see the article on Wikipedia.