Barry, the colourtune is just a spark plug with a viewing window, so you can see the flame go from yellow to blue and get the stoichiometric (sp?) ratio close. The Gunson Gastester is a 'sniffer' that goes in the tailpipe, and gives you exhaust gas readings. You'd be able to check your ratio at idle, cruise, heavy load... should allow you to get much closer.
Greg, I'll definitely take you up on that, I'm interested to see whether I've now got it about right!