Sansa's done more than learn from her captor/torturer/style icon Cersei: It looks like the student has surpassed the master. Because remember when Oberyn Martell told Tyrion how Cersei had tried and failed at emotional manipulation with him? Well, Sansa just categorically won all the emotional manipulation awards ever on Sunday's episode — and she made sure to dress accordingly.
After almost three full seasons of being a powerless hostage, Sansa's finally talking back. We first saw a glimpse of it a few weeks ago when she tearfully told Aunt Lysa that Littlefinger hadn't tried anything with her (and he hadn't... yet) — she used her admittedly tragic past to her advantage to convince Aunt Lysa that she had no designs on him.
And this week, she takes it a step further, conflating truth and fiction so very seamlessly that she's able to successfully convince a whole panel of suspicious Eyrie elders of Littlefinger's innocence. It's a masterful move (both by Sansa herself and actor, Sophie Turner), and her calculated, through-crocodile-tears-look at Littlefinger shows us that she's playing with the big leagues now.
Her subsequent scene with him ("I know what you want") adds feathers (no pun intended) to her cap, and her dress clinches it. Sansa's always been a survivor, but this is a dress that lets us know she's playing the infamously life-or-death game of thrones for keeps.