title: Gnuplot plot every nth line a specific colour
body: <p>I have a graph with several lines on it, read in from different columns in a data file.  I'd like each of the lines to be black, then every fifth line be red.  I've managed to get every fifth line be not-black but can't figure out how to specify red using the gnuplot rgbcolor variable syntax.<a href="https://i.stack.imgur.com/KvlOD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KvlOD.png" alt="Here is my current attempt"></a></p>
  
  <p>The line is use to specify these colours is:</p>
  
  <pre><code>plot for [i=2:max_col] 'data_file.txt' u 1:i:((i-1)%5==0?(255000000):0) w l lc rgbcolor variable lw 2.8
  </code></pre>
  
  <p>My question is, what do I place in the condition so that the line colour is red?  I'm using gnuplot 4.6 so the 255&lt;&lt;16 trick to get red produces an error.  </p>