Fire are of two types, hot and cool fire, hot includes the large, intense and sweeps whereas the cooler one’s are the low temperature fire (below 400 degrees) such burns protects the essence of life, the canopy.
Fire results in the combustion of organic material and oxygen. Chemistry shows us the chemical reactions but the light shows are all physics, when a flame burns cleanly like a gas flame the heat excited the molecules to release light usually para-blue from atomic transitions that’s quantum mechanics.
When the fuel is not pure and does not entirely burns like wood, coal and candle there is still some blue light at the top but a naked eye is unable to see the bluish color because it is over powered by all the particles of smoke and fire. Black body radiation process makes all objects glow with light with the color depending on their temperature.