tilemill css. super simple.

gistfile1.css

.tracks { 
  line-width: 1;
  line-opacity: 0.4;
  line-comp-op: screen;
 }

#tracks2013{ 
  line-color: #00bffa;
}

#tracks2012{
  line-color: #00fc53;
  }

#tracks2011 {
  line-color:#b11ff2;
}

#tracks2010 {
  line-color:#ff0000;
}

#tracks2009 {
  line-color:#f6710a;
}