Question

why dark inner zone is the coldest region of the candle flame

Answer 1

This is because the inner zone of candle flame undergoes incomplete combustion. Not all the carbon reacts with the oxygen there so the heat output is less. And that's why the innermost zone of the candle flame is not hot as the other zones...

Answer 2

this zone consists of unburnt wax vapour given off by the molten wax . so it is coldest part of the flame