In which zone of a candle flame:

(a) In the middle zone partial combustion of the fuel takes place, so the colour of the flame is yellow and it is moderately hot part of the flame. It is the luminous part of the flame.
(b) In the outer zone complete combustion of the fuel takes place, so colour of the flame is blue and it is the hottest part of the flame. It is the non luminous part of the flame.