this is no CSI, no evidences available, only the laws of physics

Once again, we need to make a distinction between startup backfire and exhaust flames:

1) on a cold engine, backfire happens for overpriming/bad magnetoes not delivering regular sparks at low revs; on a hot engine, it happens for the same conditions of a cold engine plus the hot exhausts/outlet valves/combustion chamber/carbon residues, who can ignite the non burnt mixture. In both cases the result will be a "cold flame"(as in relatively low temp flame) which will roar out of the exhaust (and warm up the combustion chamber a bit!) in a spectacular fashion. Backfire is more dangerous on radial engines because the flames can linger in the exhaust ring outlet and pre-ignite the mixture in the combustion chambers (blowing up cylinders/valves/spark plugs).

2) exhaust flames are the result of a correct combustion in the chamber, and because of the temperatures and pressures involved, they come out as hot flames normally (blue flames).

If you have the movie Tora Tora Tora have a look at the carrier takeoff sequence to see a good examples of blue flames in action.