Why Is the Flame of Candle Different Colors?
At first glance, a candle flame appears to be one color, a sort of yellow-orange. But if you study the flame closely, you will see three distinct parts. Inside the flame, above the wick, is a dark zone. It is made up of gas that is not yet burning because it has not reached the air, and its temperature is not high enough for it to burn.
Then, as the gas works outward, oxygen becomes available and burning starts. This produces the characteristic yellow light. At the edges of the flame, there is enough oxygen for complete burning. This is seen as a bluish light.
Flame color depends on several factors, the most important typically being black-body radiation and spectral band emission, with both spectral line emission and spectral line absorption playing smaller roles.
In the most common type of flame, hydrocarbon flames, the most important factor determining color is oxygen supply and the extent of fuel-oxygen pre-mixing, which determines the rate of combustion and thus the temperature and reaction paths, thereby producing different color hues.