title: pandas data frame, select most common value from all rows
body: <p>I have a data frame and I'd like to take from one Col1 the one most frequent colour. There are a few types of colours: Green, Yellow, Blue, Black.</p>
  
  <p>Col1 has a lot of data. Random colours. For example:</p>
  
  <p><a href="https://i.stack.imgur.com/JG1Ro.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JG1Ro.jpg" alt="enter image description here"></a></p>
  
  <p>Any idea? </p>