title: gitk will not start on Mac: unknown color name "lime"
body: <p>I've installed git on a mac via <code>brew install git</code>. When I try to start gitk I get the following error:</p>
  
  <pre><code>Error in startup script: unknown color name "lime"
      (processing "-fore" option)
      invoked from within
  "$ctext tag conf m2 -fore [lindex $mergecolors 2]"
      (procedure "makewindow" line 347)
      invoked from within
  "makewindow"
      (file "/usr/local/bin/gitk" line 12434)
  </code></pre>
  
  <p>It appears that my Mac doesn't have a color named <code>lime</code>. </p>
  
  <p>Can I add a lime color to the environment, or is there a better fix? </p>
  
  <p>The git version is 2.7.0, and the Mac is running Yosemite 10.10.5</p>