title: Specify RGB colour of contours with matplotlib
body: <p>OK, I think this is probably simple but I've searched extensively and can't find anything. What I want to do is specify the precise RGB colour values of a contour line generated with matplotlib. I know that if I do something like :</p>
  
  <pre><code>plt.contour(X,Y,Z,[0.1,0.2,0.3],colors='k')
  </code></pre>
  
  <p>then I'll get three contour levels which are all black, or if I were to substitute 'w' in place of 'k' then the contours would all be white. That's fine, but what I need to do is something like this :</p>
  
  <pre><code>plt.contour(X,Y,Z,[0.1,0.2,0.3],colors=(1.0,0.25,0.75)
  </code></pre>
  
  <p>Where the colours not chosen from some in-built presets but have RGB (or RGBA) values precisely defined by me. </p>
  
  <p>Any ideas ?</p>