title: Changing a face's direction towards light does not update its color
body: <p>I construct a simple Geometry (a triangle) between 3 vertices, in a scene with directional light. (left triangle in the below image)</p>
  
  <p>When I then change one of the vertices so that the triangle has a different angle towards the light, I want the color of of the triangle to reflect that (center triangle), but as you can see, the color remains exactly the same.</p>
  
  <p>When I create a new triangle between the updated vertices (triangle on the right), the color is properly set (as expected).</p>
  
  <p><img src="https://i.stack.imgur.com/mLl3e.png" alt="three js screenshot"></p>
  
  <p>Question: how can I get the triangle to dynamically update its color when I change one of its vertices?</p>
  
  <p>I've tried:</p>
  
  <pre><code>triangle.geometry.verticesNeedUpdate = true;
  triangle.geometry.colorsNeedUpdate = true;
  triangle.geometry.elementsNeedUpdate = true;
  triangle.material.needsUpdate = true;
  </code></pre>