title: Changing colors for decision tree plot created using export graphviz
body: <p>I am using scikit's regression tree function and graphviz to generate the wonderful, easy to interpret visuals of some decision trees:</p>
  
  <pre><code>dot_data = tree.export_graphviz(Run.reg, out_file=None, 
                           feature_names=Xvar,  
                           filled=True, rounded=True,  
                           special_characters=True) 
  graph = pydotplus.graph_from_dot_data(dot_data)
  graph.write_png('CART.png')
  graph.write_svg("CART.svg")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/vaoPn.png" rel="noreferrer"><img src="https://i.stack.imgur.com/vaoPn.png" alt="enter image description here"></a></p>
  
  <p>This runs perfectly, but I'd like to change the color scheme if possible?  The plot represents CO<sub>2</sub> fluxes, so I'd like to make the negative values green and positive brown.  I can export as svg instead and alter everything manually, but when I do, the text doesn't quite line up with the boxes so changing the colors manually and fixing all the text adds a very tedious step to my workflow that I would really like to avoid! <a href="https://i.stack.imgur.com/KXKVu.png" rel="noreferrer"><img src="https://i.stack.imgur.com/KXKVu.png" alt="enter image description here"></a></p>
  
  <p>Also, I've seen some trees where the length of the lines connecting nodes is proportional to the % variance explained by the split.  I'd love to be able to do that too if possible?</p>