Duchess Kate wore light blue skinny jeans, her black ballet shoes, I believe they are her London Sole ones and a loose fitting cream peasant top. Duchess Kate wore her hair down with sunglasses. We rarely see Kate photographed wearing sunglasses. Would love to have seen the top from the front but from what I can see it looks quite interesting. Perhaps Whistles or Reiss? She always pulls off casual chic excellently and looks just like any other young wife out shopping.
The photos were taken mostly from behind. I imagine the photographer didn't want to be too intrusive or to interfere with her day. It is excellent to see photographers respecting Kate's privacy. Today is Pippa's 28th birthday, (Happy Birthday Pippa :)) perhaps Kate was out shopping for a gift for her sister? I imagine Pippa will have a family celebration so there may be more photos very soon!
UPDATE: It was confirmed that William and Kate did indeed celebrate Pippa's 28th birthday with her younger sister and Alex Loudon. Unfortunately no photos thus far but I'll keep a look out!