When a candle is lit, paraffin wax (a hydrocarbon) vaporuizes and mixes with air to form a combustible mixture, and burns. The air-fuel ratio changes as we move away from the wick. As a result, broadly, we have three different flame regions – blue, violet and yellow.
The region immediately close to the wick is fuel rich and there is insufficient oxygen. Hence the mix does not burn and the region is dark. Outside this region, the fuel-air ratio of the mixture is right enough to burn. The blue-violet colour is due to the presence of CH radicals formed during the combustion reactions. The outer yellow region is due to the radiation from fine hot soot (carbon) particles.