Sienna Miller wore a bright blue gown out to support Matthew Williamson at his book launch party in London yesterday. As his friend and muse, Sienna hosted the event and wrote the forward, which explores the designer's body of work and history in the fashion business. Sienna recently showed off her stylish side with multiple outfits at the British Film Festival in France, where she made quite the chic judge. She'll be surrounded by more fashionable friends as the host of the upcoming Esquire House charity event in LA that will bring her and Jude Law to CA on Friday.