Hi kiwi,
Orange and yellow flames are certainly conductive and especially the smoke which is unburned fuel and mostly carbon.
Carbon is a good conductor and can trigger flasovers between phases or across insulators and possibly crack skirts off them.
Also a sustained arc may melt the actual conductor and break the areal conductor.
( like the old carbon rod lamps used in the old movieprojectors ).
If a flame is blue or even invisible, the fuel, air ratio is very good and complete burning is achieved with hardly any waste hence no colour in the flames.
The other factor to bear in mind when bush fires burn underneath transmission lines is the risk of sagging of the lines because of the heat from the fire.
