title: How to save pandas to excel with different colors
body: <p>I have a dataframe</p>
  
  <pre><code>df = pd.DataFrame(columns = ['id', 'name', 'age'])
  df.loc[len(df), :] = [1, 'John', 21]
  df.loc[len(df), :] = [2, 'Mary', 19]
  df.loc[len(df), :] = [3, 'Ann', 27]
  df.loc[len(df), :] = [4, 'Ben', 18]
  </code></pre>
  
  <p>I want to save it to excel file using <code>xlsxwriter</code>. </p>
  
  <p>However, I want the <code>age</code> larger than 20 to be in red.</p>
  
  <p>In other words, <code>21</code> and <code>27</code> should appear red in the excel file.</p>
  
  <p>How to do it?</p>