Sienna Miller is perfecting her juggling act: Two days ago — and hours before showing her Twenty8Twelve collection in London — Miller wrapped “Yellow,” where she stars as Xanne, a girl with Tourette’s syndrome. “It was a tough role — but rewarding and fun,” she said from the front row of her pal Matthew Williamson’s show on Sunday night. The film is written and directed by Nick Cassavetes, and Miller said that she trained for the part by spending time with a person who suffers from Tourette’s. In November, she’ll begin filming “The Trials of Cate McCullough,” where she’ll appear alongside Kate Beckinsale. Meanwhile, the fashion collection she designs with her sister Savannah is becoming a star of its own. Miller said it’s the best-selling brand on Selfridges’ third floor, with 70 percent sell-throughs.