title: Pandas conditional creation of a series/dataframe column
body: <p>How do I add a <code>color</code> column to the following dataframe so that <code>color='green'</code> if <code>Set == 'Z'</code>, and <code>color='red'</code> otherwise?</p>
  <pre><code>    Type       Set
  1    A          Z
  2    B          Z           
  3    B          X
  4    C          Y
  </code></pre>