title: How to redefine a color for a specific value in a matplotlib colormap
body: <p>I want to use the colormap <code>rainbow</code> in an image using <code>imshow</code>. It works perfectly, but I want to redefine the color for the value <code>0</code>. Instead of violete, I want to use white. </p>
  
  <p>I want to do this only for the value zero, all other values can stay with the default values in the colormap.</p>
  
  <p>Any idea how to do this <strong>without</strong> having to edit the colormap manually?</p>