title: ContourPlot: Styling contour lines
body: <p>I can plot the curve corresponding to an implicit equation:</p>
  
  <pre><code>ContourPlot[x^2 + (2 y)^2 == 1, {x, -1, 1}, {y, -1, 1}]
  </code></pre>
  
  <p>But I cannot find a way to color the contour line depending on the location of the point. More precisely, I want to color the curve in 2 colors, depending on whether x² + y² &lt; k or not.</p>
  
  <p>I looked into ColorFunction but this is only for coloring the region between the contour lines.
  And I was not able to get ContourStyle to accept a location-dependent expression.</p>