Borrowing from [amr][1]'s answer [here][2]


    ToExpression["16^^" <> #] & /@ 
       Partition[Characters@StringTrim[#, "#"], 2] & /@ {"FFFFFF", 
      "#FFFFFF", "000000", "#FF5500", "#005500"}

=>{{255, 255, 255}, {255, 255, 255}, {0, 0, 0}, {255, 85, 0}, {0, 85, 
  0}}

  [1]: http://mathematica.stackexchange.com/users/950/amr?tab=answers
  [2]: http://mathematica.stackexchange.com/a/11089/106