Why is the flame of the candle yellow ?

The yellow color is candle flame is due to microscopic bits of carbon glowing with heat energy. The substance that burns in a candle is vaporized wax, which is emitted by the hot wick and rises upward through the flame because it is hotter than the surrounding air. Only some of this hydrocarbon vapour is burned as it passes through the flame, the unburned portion consists of tiny particles mainly of carbon. 

These small solid carbon particles are heated by the flame to incandescence. They give off a yellow glow because their temperature is in the range that gives off yellow light. Cooler temperatures will produce orange or, even cooler, red light, and hotter temperatures produce white, or hotter still, blue-white light. Candle flames are just hot enough to produce yellow light.

After passing upward out of the flame, the particles cool off to room temperature and you see them as smoke or soot, which can deposit on surfaces held above the candle flame.