title: Different between colour argument and aes colour in ggplot2?
body: <p>When I use the colour in aes like this</p>
  
  <pre><code>ggplot(data=olympia,aes(x=year)) + geom_line(aes(y=gold,colour="red")) + geom_line(aes(y=silver,colour="blue"))
  </code></pre>
  
  <p>it does not work.</p>
  
  <p>If I use the colour argument it shows the right colours red and blue</p>
  
  <pre><code>ggplot(data=olympia,aes(x=year)) + geom_line(aes(y=gold),colour="red") + geom_line(aes(y=silver),colour="blue")
  </code></pre>
  
  <p>What is the different?
  What's the fault?</p>
  
  <p>Dataframe</p>
  
  <pre><code>year gold silver
  1 2002   12     16
  2 2006   11     12
  3 2010   10     13
  4 2014    8      3
  </code></pre>