Complex sentence: The producers imagined Darcy to be dark despite no such references in the novel and asked Firth to dye black his light-brown hair, eyebrows and lashes; they instructed all male actors to let their hair grow before filming and shave off their moustaches.
Split into simpler sentences: The producers imagined Darcy to be dark despite no such references in the novel, and asked Firth to dye his naturally light-brown hair, his eyebrows and lashes, black. They additionally instructed all male actors to let their hair grow before filming began and warned them to shave off their moustaches.