It's no secret style-conscious celebrities love to wear Matthew Williamson — his most loyal supporters include chic hipsters such as Sienna Miller, Sophie Dahl, Jade Jagger and Jacquetta Wheeler. But this summer, one purple number has proven to be a clear favorite among the Hollywood set. The filmy Empire frock has graced a slew of ingenues, most recently Keira Knightley at the London premiere of her new film, "Pride & Prejudice." Knightley, who is styled by the ubiquitous Rachel Zoe, accessorized her long gown with new honey-hued extensions. The usually style-challenged Tara Reid also wore the look — in a refreshing break from her string bikinis and mini minis — at London's Pangaea club during a whirlwind European tour this summer while filming the E network's "Taradise." "Wedding Crashers" star Isla Fisher was the first to wear the shorter version, chosen by her stylist, Jessica Paster, from Williamson's resort collection, at the New York premiere of the hit film.