Question

why the middle zone of a flame is yellowish in colour

Answer 1

It contains low oxygen in it as the higher or top of the blue flame is the red or fire like flame which contains more oxygen and oxidises the compound or components more than the starting or initial points.

Answer 2

not necessarily,in an oxidising flame,it's blue.....all this depends on the element kept on the flame as different elements change the flame colour accordingly... yellow colour of the flame is due to the reducing flame i.e. due to carbons or hydrocarbon compound.