Sienna Miller stepped out in a summery green dress after the Cox, Cookies and Cake store opening in London on Sept. 1.

The actress spent the day with her sister and co-designer of Twenty8Twelve, Savannah Miller.

Sienna piled on the accessories: fringe earrings, glitzy belt, cuff bracelet and box clutch with a kiss clasp. We're glad she kept the shoes simple with black pumps.

Her minimal beauty - virtually zero make up and bare nails - is why Sienna is the definition of effortless glam.

Update: The brit was wearing her own design! The green dress - in addition to the washed denim jacket her sister wore that evening - was Twenty8Twelve.

In related news, Nicole Richie is back to blond! Check out her new do.