title: how to subtract string type columns values from another column in pandas
body: <p>I have a data frame like this </p>
  
  <pre><code>df
  col1         col2          col3
   A        black berry      black
   B        green apple      green
   C        red wine          red
  </code></pre>
  
  <p>I want to subtract col3 values from col2 values, the result will look like</p>
  
  <pre><code>df1
  col1        col2        col3
    A         berry       black
    B         apple       green
    C          wine        red
  </code></pre>
  
  <p>How to do it in effective way using pandas</p>