Why is the flame of the candle yellow in colour?
The flame of a burning candle is dominated by yellow color although there are other color regions in particular blue color in a small way.
In the flame, mechanism(s) of light production for different regions differ and determined by the ambient atmosphere during the course of candle burning (that is presence or absence of oxygen/air, hydrogen from the break-down of hydrocarbons, the usual constituent of the candle material).
As the candle wick is lighted, it picks up the fuel through capillary action of the wick from the immediate surrounding region of the burning wick.
This molten region of the paraffin/wax-hydrocarbons gets burnt and undergoes molecular transitions determined by the environ.
The miniscule blue region of the flame can be attributed to hydrogen burning resulting from the break-down of hydrocarbons from the candle stuff while the incandescent wick material, the carbonaceous materials such as soot from the burning of candle stuff, with limited supply of oxygen dominating the scenario results in a yellow flame . The latter process can be better described by incandescence, a process of light generation through burning analogizing glow of tungsten filament in an electrical bulb or glow of lighted match stick and so on yielding yellow flames where most of the energy is dissipated as heat radiation.
It should be borne in mind that a blue flame yields higher temperate than does a yellow flame.
Also it is common experience to see a domestic gas stove showing difference in flame color changing from yellowish blue to deep blue as we change the control knob ( letting air through nozzle determining the speed of combustion of the gas) from sim mode to high flame mode.
Karaikudi, Tamil Nadu