ORIGINAL SOURCE: Keira Knightley continues to appear in terrific shades of blue - one of this new Fall season's hottest colors. Who can forget the gorgeous cornflower blue Alexis Mabille couture creation at the London premiere?

During her guest appearance on The Late Show with David Letterman on September 8th to promote The Duchess, she wore a prim, classic marine silk floral print dress by 3.1 Phillip Lim. Upon leaving the theatre, she added a 3/4 length sleeve jacket, shown here, but the winning look was on the show. This is the kind of dress that will be a staple in your closet, a perfect item for those times when you need to "turn day into night" .. Keira did just that!

The dress is available online at Net-A-Porter - it's very easy to "Get The Look of Keira Knightley", more photos & link to buy the dress below...