Yellow Flame.

I recall that the discussion was along the lines of the colour of the flame, i.e. yellow flame bad blue flame good, but no more detail than that.

I will ask him next time I see him.

Incidently my 123R normally does burn with a slight tinge of yellow on the flame tips, but my recently aquired Coleman Sportstar II burns with a clean blue flame within 30 secs or so.