http://forum.arduino.cc/index.php?topic=140308.0
http://forum.arduino.cc/index.php?topic=89104.45
As far as I can tell , the 18 outputs need to grouped as 6 groups of 3 leds each (R,G,B) so:
OUT1=R
OUT2=G
OUT3=B
OUT4=R
OUT5=G
OUT6=B
etc
etc
etc