title: modify pandas column value based on another column
body: <p>Here is my dataframe:</p>
  
  <pre><code>                      Nets  y_start  draw_length color bar_label  label_pos
  2018-07-30 06:04:10  11319      0.0      11319.0  grey    11,319      11319
  2018-08-20 08:11:45  21889  11319.0      10570.0  grey    21,889      21889
  2018-09-10 08:39:19  16270  21889.0      -5619.0   red    16,270      16270
  </code></pre>
  
  <p>I need to subtract 10000 from the existing lable_pos if the color is red.</p>
  
  <p>I tried this but I get invalid syntax-</p>
  
  <pre><code>df['label_pos'] = df['lable_pos'] - 10000 if df['color'].isin('red')
  </code></pre>
  
  <p>what I'm after is the last row in this example is: </p>
  
  <pre><code>2018-09-10 08:39:19  16270  21889.0      -5619.0   red    16,270      15270
  </code></pre>