System.Windows.Controls.Button b = new System.Windows.Controls.Button();
b.Content = "OK";
b.Background = System.Windows.Media.Brushes.Red;

For more WPF interview question please check this link...
http://mindstick.com/Interview/401/How%20to%20change%20backcolor%20of%20button%20control%20dynamically

There is no Burshes class it is Brushes class............

To change the background color of a button dynamically in WPF

System.Windows.Controls.Button b1=new System.Windows.Controls.Button();

b1.Background=System.Windows.Media.Brushes.Blue;

System.Windows.Controls.Button objButtton=new System.Windows.Controls.Button();

objButton.Background=System.Windows.Media.Burshes.Red;