title: How to change one line color in geom_line
body: <p>I have 24 groups that I have plotted using geom_line in ggplot2.  I was wondering whether there is a quick way to change the colour of one of those particular groups to black without specifying the colours of the other 23?</p>
  
  <p>My code:</p>
  
  <pre><code>ggplot(max, aes(x = dap,y = cf, group=variables, colour = variables)) +
                    geom_line(size=0.5) 
  </code></pre>