(The). The 35th Foot. Called “orange” because their
facings were orange till 1832; and “lilies” because they were
given white plumes in recognition of their gallantry in the battle of
Quebec in 1759, when they routed the Royal Roussillon French
Grenadiers. The white plume was discontinued in 1800. The 35th Foot is
now called the “The Royal Sussex.”
William of Orange.
William III. of England (1650, 1689-1702). “Orange” is a
corruption of Arausio, in the department of Vaucluse, some sixteen
miles from Avignon. The town was the capital of a principality from the
eleventh to the sixteenth century. The last sovereign was Philibert de
Chaons, whose sister married William, Count of Nassau. William's
grandson (William) married Mary, eldest daughter of Charles I., and
their eldest son was our William III., referred to in the text.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Orange Lilies from Infoplease: